GS Cache: refactor files iteration.

This change refactors files iteration in response of multi-part range
request using coroutine. This will make code easier to understand and
maintain.

BUG=chromium:824580
TEST=Ran unit tests.

Change-Id: I3a261855245e37f6f1a6171369cb9fa27cb5ed70
Reviewed-on: https://chromium-review.googlesource.com/1108797
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Congbin Guo <guocb@chromium.org>
Reviewed-by: Congbin Guo <guocb@chromium.org>
2 files changed