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Previously, every gyp file that wanted to set clang warnings had to check
for clang==1 and then set cflags and xcode_settings.WARNING_CFLAGS. Factor
this out, so that targets only need to set clang_warning_flags for warnings
that apply to all platforms. (Per-platform flags still need to be set manually.)

This removes existing duplication from gyp files, and prevents adding more
duplication when trying to add the same warning flags for clang/win.

BUG=82385
R=hans@chromium.org, scottmg@chromium.org
TBR=various owners

Review URL: https://codereview.chromium.org/437543007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@287092 0039d316-1c4b-4281-b951-d872f2087c98