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Currently we place a 2-second on URLRequestContextAdapter initialization, so
that context can be initialized when embedders make requests. This CL changes
so that tasks that depend on context being initialized will be run after the
context is initialized properly, and URLRequestContextAdapter initialization
won't block.

BUG=

Review URL: https://codereview.chromium.org/617393005

Cr-Commit-Position: refs/heads/master@{#299145}