Fix themes on retina Mac

Fix themes on retina Mac

Make BrowserThemePack stop assuming that the maximum persistent ID is related to
the the size of kPersistingImages. https://codereview.chromium.org/453213002
broke this assumption on Mac. This assumption has been broken on Windows for a
long time

BUG=405105
TEST=None

Committed: https://crrev.com/e5011438e2c769c831b39e9665b95658ca0932eb
Cr-Commit-Position: refs/heads/master@{#293406}

[pkotwicz, 2014-09-04 21:13:05]

pkotwicz@chromium.org changed reviewers:
+ erg@chromium.org

[pkotwicz, 2014-09-04 21:13:05]

Elliot, can you please take a look?

[pkotwicz, 2014-09-04 21:14:45]

I have introduced GetMaxPersistentId() instead of using a constant because I
think that this is less likely to be broken

[Elliot Glaysher, 2014-09-04 21:20:51]

lgtm

[pkotwicz, 2014-09-04 21:29:43]

The CQ bit was checked by pkotwicz@chromium.org

[commit-bot: I haz the power, 2014-09-04 21:32:13]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/pkotwicz@chromium.org/542833002/1

[commit-bot: I haz the power, 2014-09-05 02:28:17]

Committed patchset #1 (id:1) as f5ffcb5d3ecd197523dfd37fc12a75654615ffd4

[commit-bot: I haz the power, 2014-09-10 03:35:45]

Patchset 1 (id:??) landed as
https://crrev.com/e5011438e2c769c831b39e9665b95658ca0932eb
Cr-Commit-Position: refs/heads/master@{#293406}