[ChromeDriver] Fix LATEST_RELEASE update code

A misspelled platform name was preventing build bots from automatically
updating LATEST_RELEASE file.

Change-Id: I752b243fce97fff5776c7bec33cb6b2031e39508
Reviewed-on: https://chromium-review.googlesource.com/1017000Reviewed-by: Jonathon Kereliuk <kereliuk@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551876}

chrome/test/chromedriver/run_buildbot_steps.py

@@ -324,7 +324,7 @@ def _MaybeUpdateLatestRelease(version):
     return
 
   # Check if chromedriver was released on all supported platforms.
-  supported_platforms = ['linux64', 'mac32', 'win32']
+  supported_platforms = ['linux64', 'mac64', 'win32']
   for platform in supported_platforms:
     if not _WasReleased(version, platform):
       return