Add skia_clang_build gyp define, add -Wstring-conversion flag

If you want the warning, you have to specify skia_clang_build=1

R=djsollen@google.com

Review URL: https://codereview.chromium.org/23038014

git-svn-id: http://skia.googlecode.com/svn/trunk@10861 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed