Move enable_base_tracing/use_perfetto_client_library to build_overrides.

This will allow projects that use //base to override
these two GN args in an easier way (//build_overrides/build.gni
instead of gn args).

Bug: 1093406
Change-Id: I26d012732a428798a28e43726ce0abba54b3dd68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2241123
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#777444}

base/BUILD.gn

@@ -18,7 +18,6 @@
 # huge sequence of random-looking conditionals.
 
 import("//base/allocator/allocator.gni")
-import("//base/trace_event/features.gni")
 import("//build/buildflag_header.gni")
 import("//build/config/allocator.gni")
 import("//build/config/arm.gni")
@@ -34,6 +33,7 @@ import("//build/config/sysroot.gni")
 import("//build/config/ui.gni")
 import("//build/nocompile.gni")
 import("//build/timestamp.gni")
+import("//build_overrides/build.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 import("//testing/test.gni")
 import("//third_party/icu/config.gni")

base/test/BUILD.gn

@@ -2,10 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//base/trace_event/features.gni")
 import("//build/compiled_action.gni")
 import("//build/config/nacl/config.gni")
 import("//build/config/ui.gni")
+import("//build_overrides/build.gni")
 
 # Reset sources_assignment_filter for the BUILD.gn file to prevent
 # regression during the migration of Chromium away from the feature.

build_overrides/build.gni

@@ -2,7 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//base/trace_event/features.gni")
 import("//build/config/gclient_args.gni")
 
 # Uncomment these to specify a different NDK location and version in
@@ -13,6 +12,22 @@ import("//build/config/gclient_args.gni")
 # Some non-Chromium builds don't support building java targets.
 enable_java_templates = true
 
+# Features used by //base/trace_event and //services/tracing.
+declare_args() {
+  # Tracing support requires //third_party/perfetto, which is not available in
+  # libchrome (CrOS's version of //base). This flag can disable tracing support
+  # altogether, in which case all tracing instrumentation in //base becomes a
+  # no-op.
+  # TODO(crbug/1065905): Add dependency on perfetto to support typed events.
+  enable_base_tracing = true
+
+  # Switches the TRACE_EVENT instrumentation from base's TraceLog implementation
+  # to //third_party/perfetto's client library. Not implemented yet, currently a
+  # no-op to set up trybot infrastructure.
+  # TODO(crbug/1006769): Switch to perfetto's client library.
+  use_perfetto_client_library = false
+}
+
 # Allows different projects to specify their own suppressions and blacklist
 # files for sanitizer tools.
 # asan_suppressions_file = "path/to/asan_suppressions.cc"

services/tracing/public/cpp/BUILD.gn

@@ -2,10 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//base/trace_event/features.gni")
 import("//build/buildflag_header.gni")
 import("//build/config/chromecast_build.gni")
 import("//build/config/compiler/compiler.gni")
+import("//build_overrides/build.gni")
 import("//services/tracing/public/cpp/stack_sampling/loader_lock_sampling.gni")
 
 buildflag_header("buildflags") {