Refactor how clang warning flags are set.

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

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: dcbc32cc8063d329ba9128807b05d912d7d3679e