Remove the old cache dir before moving the bootstrapped cache dir into the real location

Remove the old cache dir before moving the bootstrapped cache dir into the real location

Seems like some combination of things (bad cache dir, shallow checkout, etc) causes
git cache to not clean up properly before trying to move new directory A into
already existing directory B.

So the logic is all there to detect that the cache is invalid, its just not
being cleaned up properly during shallow checkouts.

BUG=406864
TBR=iannucci

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=291587

[tandrii(chromium), 2014-08-24 11:59:42]

lgtm

[Ryan Tseng, 2014-08-24 12:01:33]

The CQ bit was checked by hinoka@google.com

[commit-bot: I haz the power, 2014-08-24 12:02:16]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/hinoka@chromium.org/499123002/1

[commit-bot: I haz the power, 2014-08-24 12:02:17]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-08-24 12:02:17]

No LGTM from a valid reviewer yet. Only full committers are accepted.
Even if an LGTM may have been provided, it was from a non-committer or
a provisional committer, _not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[Ryan Tseng, 2014-08-24 12:06:15]

The CQ bit was checked by hinoka@google.com

[commit-bot: I haz the power, 2014-08-24 12:07:15]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/hinoka@chromium.org/499123002/1

[commit-bot: I haz the power, 2014-08-24 12:09:15]

Committed patchset #1 (1) as 291587