[ChromeDriver] Mitigate Mac builder timeout issue

The Mac builder for ChromeDriver often times out while waiting for
a Chromium build to become available, because the Mac Chromium
builds are very slow. Mitigate this issue by allowing slightly
older Chromium builds on Mac.

Bug: chromedriver:2584
Change-Id: Id219859252b11cb7ec450bdef87314a107c49fef
Reviewed-on: https://chromium-review.googlesource.com/1220400
Commit-Queue: John Chen <johnchen@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590930}

chrome/test/chromedriver/run_buildbot_steps.py

@@ -346,9 +346,14 @@ def _MaybeUpdateLatestRelease(version):
 
 def _WaitForLatestSnapshot(commit_position):
   util.MarkBuildStepStart('wait_for_snapshot')
+  assert commit_position is not None, 'Missing commit_position'
+  if util.IsMac():
+    # https://bugs.chromium.org/p/chromedriver/issues/detail?id=2584
+    # Mac Chromium builds are slow. Allow slightly out-of-date builds.
+    commit_position = int(commit_position) - 100
   for attempt in range(0, 200):
     snapshot_position = archive.GetLatestSnapshotPosition()
-    if commit_position is not None and snapshot_position is not None:
+    if snapshot_position is not None:
       if int(snapshot_position) >= int(commit_position):
         break
       util.PrintAndFlush('Waiting for snapshot >= %s, found %s' %