Merge 317853 - Make BackgroundContentsService start up BackgroundContents with a delay, as for Exte…

Merge 317853 - Make BackgroundContentsService start up BackgroundContents with a delay, as for ExtensionHosts.

extensions::ProcessManager starts ExtensionHosts asynchronously with a queue to avoid using too much resources at startup; make BackgroundContentsService use the same queue implementation. (ExtensionHostQueue now accepts an interface that both BackgroundContentsService and ExtensionHost share.)

This significantly reduces the time BackgroundContentsService contributes to ExtensionService startup (75th percentile was ~300ms, according to Extensions.BackgroundContentsServiceStartupTime).

BUG=47236
Review URL: https://codereview.chromium.org/933423003

Cr-Commit-Position: refs/heads/master@{#317480}
(cherry picked from commit d61dfe19ab3188101e511f633eeaaef3dc689e41)

Committed: https://chromium.googlesource.com/chromium/src/+/4bda951ed28b1e47ae0de179bbc03aad62b92b99

[Yoyo Zhou, 2015-02-24 21:42:43]

Committed patchset #1 (id:1) manually as
4bda951ed28b1e47ae0de179bbc03aad62b92b99.