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This is a prequisite for more additions to ScopedBlockingCall's
internal namespace for the IO jank metric. It was getting too big and
polluting the header. Extracting an _internal.h header is the common
paradigm to work around this.

R=fdoray@chromium.org, olivierli@google.com

Bug: 1064645
Change-Id: I0b3c49222026d2d98db77eb6075efcf17d9f33e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2102588
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753456}