Use third_party clang on Mac.

BUG=

Review URL: https://codereview.chromium.org/1200833013

Cr-Commit-Position: refs/heads/master@{#29343}

build/standalone.gypi

@@ -771,8 +771,11 @@
           }],
           ['clang==1', {
             'xcode_settings': {
+              'CC': '<(clang_dir)/bin/clang',
+              'LDPLUSPLUS': '<(clang_dir)/bin/clang++',
               'GCC_VERSION': 'com.apple.compilers.llvm.clang.1_0',
               'CLANG_CXX_LANGUAGE_STANDARD': 'gnu++0x',  # -std=gnu++0x
+              'CLANG_CXX_LIBRARY': 'libc++',
             },
           }],
         ],