GitLab

Add caching for ContentHash instances for a particular version of
an extension. Since ContentHash is required for all extension
resource requests when content verification applies, this should
speed up ContentVerifyJob.

Note that we should expect gains in
"ExtensionContentVerifyJob.TimeSpentUS" UMA.

Bug: 796395
Test: No visible behavior changes expected.
Change-Id: I4d03bcd95f00215628eb9b3f143e15e7bd2c84d5
Reviewed-on: https://chromium-review.googlesource.com/c/1266604
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605875}