Define SDKROOT for Mac

otherwise, the clang we ship doesn't work

BUG=none
R=adamk@chromium.org,thakis@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/792783006

Cr-Commit-Position: refs/heads/master@{#25893}

build/standalone.gypi

@@ -386,6 +386,7 @@
     }],  # OS=="win"
     ['OS=="mac"', {
       'xcode_settings': {
+        'SDKROOT': 'macosx',
         'SYMROOT': '<(DEPTH)/xcodebuild',
       },
       'target_defaults': {