[ChromeDriver] Disabling GPU for Mac platform

The waterfall tests on Mac appear to be flaky so disabling GPU for Mac platforms as well.

Bug: chromedriver:1695
Change-Id: Id39fea14a40857bec33e78ffe2d3c65e51ada91c
Reviewed-on: https://chromium-review.googlesource.com/884303
Commit-Queue: Artur Khachatryan <khachatryan@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531669}

chrome/test/chromedriver/client/chromedriver.py

@@ -152,7 +152,8 @@ class ChromeDriver(object):
     elif chrome_binary:
       options['binary'] = chrome_binary
 
-    if sys.platform.startswith('linux') and android_package is None:
+    if (sys.platform.startswith('linux') or sys.platform.startswith('darwin')) \
+        and android_package is None:
       if chrome_switches is None:
         chrome_switches = []
       # Workaround for crbug.com/611886.