Remove dependency on GYP_DEFINES in clang gclient hook

The gold plugin is needed only for linux official builds. However, since
it's only 17MB, it's better to always download it rather than having it
depend on GYP_DEFINES.

BUG=570091, 699210

Review-Url: https://codereview.chromium.org/2733043002
Cr-Commit-Position: refs/heads/master@{#455991}

tools/clang/scripts/update.py

@@ -397,9 +397,8 @@ def VeryifyVersionOfBuiltClangMatchesVERSION():
 def UpdateClang(args):
   print 'Updating Clang to %s...' % PACKAGE_VERSION
 
-  need_gold_plugin = 'LLVM_DOWNLOAD_GOLD_PLUGIN' in os.environ or (
-      sys.platform.startswith('linux') and
-      'buildtype=Official' in os.environ.get('GYP_DEFINES', ''))
+  # Required for LTO, which is used when is_official_build = true.
+  need_gold_plugin = sys.platform.startswith('linux')
 
   if ReadStampFile() == PACKAGE_VERSION and not args.force_local_build:
     print 'Clang is already up to date.'