Revert 284930 "Use libc++ for all non-Windows ASan builds."

Broke Mac ASan Builder gclient runhooks step:

Error: Command /Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/python src/build/gyp_chromium returned non-zero exit status 1 in /Volumes/data/b/build/slave/Mac_ASan_Builder/build
Updating projects from gyp files...
Using overrides found in /Users/chrome-bot/.gyp/include.gypi
gyp: Found /Volumes/data/b/build/slave/Mac_ASan_Builder/build/src/third_party/libc++/libc++.gyp:libcxx_proxy#target in dependencies! of /Volumes/data/b/build/slave/Mac_ASan_Builder/build/src/base/base.gyp:sanitizer_options#target, but not in dependencies
builder -> 'ninja'
gyp_defines -> {'asan': '1', 'gomadir': '/Volumes/data/b/build/goma', 'use_goma': '1', 'component': 'static_library', 'release_extra_cflags': '-gline-tables-only'}
IsWindows -> False
IsLinux -> False
platform -> 'mac'
Running build/landmines.py...
Hook '/Library/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/MacOS/python src/build/gyp_chromium' took 51.21 secs

> Use libc++ for all non-Windows ASan builds.
> 
> BUG=318770
> TBR=earthdok@chromium.org
> 
> Review URL: https://codereview.chromium.org/414703002

TBR=glider@chromium.org

Review URL: https://codereview.chromium.org/410213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@284932 0039d316-1c4b-4281-b951-d872f2087c98

build/common.gypi

@@ -2086,7 +2086,7 @@
         'clang%': 1,
         'use_allocator%': 'none',
       }],
-      ['asan==1 and OS!="win"', {
+      ['asan==1 and OS=="linux"', {
         'use_custom_libcxx%': 1,
       }],
       ['ubsan==1', {