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Implementations of WorkerLoaderProxyProvider::postTaskToLoader have used
ExecutionContext::postTask to post tasks to the main thread. However,
the posted tasks are unexpectedly throttled, and that delays the startup
time of service workers.
This CL replaces ExecutionContext::postTask there with ParentFrameTaskRunners
and WebTaskRunner::postTask, so that the posted task is routed to suitable
task runners.

BUG=667310, 671084

Review-Url: https://codereview.chromium.org/2574703002
Cr-Commit-Position: refs/heads/master@{#438146}