Revert "[ChromeDriver] Mitigate Mac builder timeout issue"

This reverts commit 887f2042.

Reason for revert: The underlying cause of the timeout (issue #883609)
has been resolved, so this mitigation is no longer necessary.

Original change's description:
> [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}

TBR=crouleau@chromium.org,johnchen@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: chromedriver:2584
Change-Id: Ic4b0440c672dc94e0fba366f2f705377a11341a7
Reviewed-on: https://chromium-review.googlesource.com/1233120Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592323}

chrome/test/chromedriver/run_buildbot_steps.py

@@ -346,14 +346,9 @@ 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 Allow slightly out-of-date builds.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 snapshot_position is not None:
+    if commit_position is not None and snapshot_position is not None:
       if int(snapshot_position) >= int(commit_position):
         break
       util.PrintAndFlush('Waiting for snapshot >= %s, found %s' %