Set clang directory in gyp.

BUG=

Review URL: https://codereview.chromium.org/764803002

Cr-Commit-Position: refs/heads/master@{#25553}

build/standalone.gypi

@@ -33,6 +33,8 @@
   'includes': ['toolchain.gypi'],
   'variables': {
     'component%': 'static_library',
+    'make_clang_dir%': '../third_party/llvm-build/Release+Asserts',
+    'clang_xcode%': 0,
     'asan%': 0,
     'tsan%': 0,
     'visibility%': 'hidden',
@@ -432,5 +434,20 @@
         ],  # target_conditions
       },  # target_defaults
     }],  # OS=="mac"
+    ['clang==1 and ((OS!="mac" and OS!="ios") or clang_xcode==0) '
+        'and OS!="win"', {
+      'make_global_settings': [
+        ['CC', '<(make_clang_dir)/bin/clang'],
+        ['CXX', '<(make_clang_dir)/bin/clang++'],
+        ['CC.host', '$(CC)'],
+        ['CXX.host', '$(CXX)'],
+      ],
+    }],
+    ['clang==1 and OS=="win"', {
+      'make_global_settings': [
+        # On Windows, gyp's ninja generator only looks at CC.
+        ['CC', '<(make_clang_dir)/bin/clang-cl'],
+      ],
+    }],
   ],
 }