If Oilpan is enabled, warn of raw heap pointer fields by default.

It is unsafe to keep fields with raw pointers into the Oilpan heap, as
such untraced references risk going stale. With potentially undesirable
consequences.

Now that we've addressed and handled all such untraced references as part
of Blink's transition to Oilpan, it is time to enable the clang GC plugin
warning for such raw pointer uses.

It shouldn't represent a major imposition to developers to handle such
raw pointer uses correctly, but for now we will only emit a warning and
not an error.

R=haraken
BUG=515524

Review URL: https://codereview.chromium.org/1464293002

Cr-Commit-Position: refs/heads/master@{#361656}

tools/clang/scripts/blink_gc_plugin_flags.py

@@ -15,18 +15,27 @@
 
 FLAGS = '-Xclang -add-plugin -Xclang blink-gc-plugin'
 PREFIX= ' -Xclang -plugin-arg-blink-gc-plugin -Xclang '
+
+warn_raw_pointers = None
 for arg in sys.argv[1:]:
   if arg == 'enable-oilpan=1':
     FLAGS += PREFIX + 'enable-oilpan'
+    if warn_raw_pointers is None:
+      warn_raw_pointers = True
   elif arg == 'dump-graph=1':
     FLAGS += PREFIX + 'dump-graph'
   elif arg == 'warn-raw-ptr=1':
-    FLAGS += PREFIX + 'warn-raw-ptr'
+    warn_raw_pointers = True
+  elif arg == 'warn-raw-ptr=0':
+    warn_raw_pointers = False
   elif arg == 'warn-unneeded-finalizer=1':
     FLAGS += PREFIX + 'warn-unneeded-finalizer'
   elif arg.startswith('custom_clang_lib_path='):
     CLANG_LIB_PATH = arg[len('custom_clang_lib_path='):]
 
+if warn_raw_pointers is True:
+  FLAGS += PREFIX + 'warn-raw-ptr'
+
 if not sys.platform in ['win32', 'cygwin']:
   LIBSUFFIX = 'dylib' if sys.platform == 'darwin' else 'so'
   FLAGS = ('-Xclang -load -Xclang "%s/libBlinkGCPlugin.%s" ' + FLAGS) % \