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This change
* modifies PpdProvider to fail all outstanding deferred callbacks each
  time it fails to obtain a metadata locale,
* adds a slowdown mechanism to its subsequent attempts to obtain a
  metadata locale,
* modifies PpdProvider to fail the oldest deferred method with
  PpdProvider::CallbackResultCode::SERVER_ERROR (rather than
  INTERNAL_ERROR) to reflect that methods are being enqueued faster
  than the hypothetical server can respond, and
* updates the method deferral unit tests to match.

    --gtest_filter='PpdProviderTest.FailsOldestQueued*'

Bug: chromium:888189
Test: chromeos_unittests \
Change-Id: Ib3ffbc81e8174f5222a7c55535ff9fc25bc7823d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303979
Commit-Queue: Kalvin Lee <kdlee@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791556}