Merge to XFA: (Reland) Switch builds to clang by default for Linux and OS X.

DEPS

 deps = {
   "build/gyp":
     "https://chromium.googlesource.com/external/gyp",
 
   "buildtools":
     "https://chromium.googlesource.com/chromium/buildtools.git@46ce8cb60364e9e0b21a81136c7debdddfd063a8",
 
   "testing/corpus":
      "https://pdfium.googlesource.com/pdfium_tests@4b9a1d593e5a101d034a4f1195174a22c179a42d",
 
   "testing/gmock":
      "https://chromium.googlesource.com/external/googlemock.git@29763965ab52f24565299976b936d1265cb6a271",
 
   "testing/gtest":
      "https://chromium.googlesource.com/external/googletest.git@8245545b6dc9c4703e6496d1efd19e975ad2b038",
 
+  "tools/clang":
+    "https://chromium.googlesource.com/chromium/src/tools/clang",
+
   "v8":
     "https://chromium.googlesource.com/v8/v8.git@4d03c3aabad6517ff058124bb799ca6a4156e570",
 
   "v8/third_party/icu":
     "https://chromium.googlesource.com/chromium/deps/icu46",
 }
 
 deps_os = {
   "win": {
     "v8/third_party/cygwin":
@@ skipping 40 matching lines @@
     'name': 'clang_format_linux',
     'pattern': '.',
     'action': [ 'download_from_google_storage',
                 '--no_resume',
                 '--platform=linux*',
                 '--no_auth',
                 '--bucket', 'chromium-clang-format',
                 '-s', 'pdfium/buildtools/linux64/clang-format.sha1',
     ],
   },
+  {
+    # Pull clang if needed or requested via GYP_DEFINES.
+    'name': 'clang',
+    'pattern': '.',
+    'action': ['python', 'pdfium/tools/clang/scripts/update.py', '--if-needed'],
+  },
 ]