add snappy and leveldatabase to the list of targets to be checked by …

…"gn check"

Checked on Linux with the following command line:

$ gn check --force gn-ninja-out/local-dbg --args='is_debug=true
is_component_build=true symbol_level=1' //third_party/leveldatabase/*

$ gn check --force gn-ninja-out/local-dbg --args='is_debug=true
is_component_build=true symbol_level=1' //third_party/snappy/*

BUG=502447

Change-Id: I6bba055a2d8b7af6aed8f1ed51ceff3140ecda2a
Reviewed-on: https://chromium-review.googlesource.com/508732
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Thiago Farina <tfarina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#472823}

.gn

@@ -143,9 +143,11 @@ check_targets = [
   #"//third_party/*",  # May not ever want this.
   "//third_party/brotli/*",
   "//third_party/hunspell/*",
+  "//third_party/leveldatabase/*",
   "//third_party/libaddressinput/*",
   "//third_party/libphonenumber/*",
   "//third_party/libwebp/*",
+  "//third_party/snappy/*",
   "//third_party/WebKit/*",
   "//tools/*",