[Merge to M38] Fix themes on retina Mac
[Merge to M38] 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
TBR=pkotwicz

Review URL: https://codereview.chromium.org/542833002

Cr-Commit-Position: refs/heads/master@{#293406}
(cherry picked from commit e5011438e2c769c831b39e9665b95658ca0932eb)

Review URL: https://codereview.chromium.org/578973002

Cr-Commit-Position: 0d344d3782ceeadf0e8e8be3f4ce70193b218c54@{#382}
Cr-Branched-From: 2cb3f0147827a1600e8cbbcc52ca24c0febcb7a4@{#290040}

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