Two options:

- simplest: just run all blob bytes transfer related code on a dedicated MayBlock SequencedTaskRunner.

- more complicated: run the main BytesProvider implementation on the IPC thread (like in the IPC based codebase), and switch to a different MayBlock task runner for file operations (old code uses FileThreadTaskRunner, but dedicated (Sequenced)TaskRunner is probably preferred now.

Benefits of the more complicated approach would be that some thread hops can be avoided for the DataPipe and Reply cases (as incoming mojo messages always pass through the IPC thread), but it's unclear if the added complexity actually makes a real impact on performance.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/8c2764154c196bf55a141141914255ff866b1c12

commit 8c2764154c196bf55a141141914255ff866b1c12
Author: Marijn Kruisselbrink <mek@chromium.org>
Date: Sat Jul 29 06:11:21 2017

Move BlobBytesProvider to a File task runner.

Move it off the main/worker thread to prevent deadlock when the main or
worker thread tries to synchronously read the blob (using XHR or
FileReaderSync). Additionally BlobBytesProvider might do blocking
file operations, which shouldn't be done on the main thread anyway, so
this moves the code to always run on the File thread. In the future it
might make sense to have part of BlobBytesProvider on the IO thread with
only the file operations on the File thread (matching the old IPC based
implementation), but for now doing all its work on the File thread should
work just fine.

This also required actually exposing a File task runner in blink::Platform.
This file task runner is not shared with the file thread in content, but
then the content one is only created on demand (for ppapi and MHTML saving)
anyway, so in most cases this should not result in extra threads.

Also add a virtual test suite to enable FileAPI (blob) layout and WPT tests
with mojo blobs, now that creating blobs works.

Bug:  740584 ,  744774 
Change-Id: Icc1e22f662dea895e1d1b2f1a1cc79c885e6706e
Reviewed-on: https://chromium-review.googlesource.com/565641
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490636}
[modify] https://crrev.com/8c2764154c196bf55a141141914255ff866b1c12/content/renderer/render_thread_impl.cc
[modify] https://crrev.com/8c2764154c196bf55a141141914255ff866b1c12/content/renderer/render_thread_impl.h
[modify] https://crrev.com/8c2764154c196bf55a141141914255ff866b1c12/third_party/WebKit/LayoutTests/NeverFixTests
[modify] https://crrev.com/8c2764154c196bf55a141141914255ff866b1c12/third_party/WebKit/LayoutTests/VirtualTestSuites
[add] https://crrev.com/8c2764154c196bf55a141141914255ff866b1c12/third_party/WebKit/LayoutTests/virtual/mojo-blobs/external/wpt/FileAPI/README.txt
[add] https://crrev.com/8c2764154c196bf55a141141914255ff866b1c12/third_party/WebKit/LayoutTests/virtual/mojo-blobs/fast/files/README.txt
[modify] https://crrev.com/8c2764154c196bf55a141141914255ff866b1c12/third_party/WebKit/Source/platform/CrossThreadCopier.h
[modify] https://crrev.com/8c2764154c196bf55a141141914255ff866b1c12/third_party/WebKit/Source/platform/blob/BlobBytesProvider.cpp
[modify] https://crrev.com/8c2764154c196bf55a141141914255ff866b1c12/third_party/WebKit/Source/platform/blob/BlobData.cpp
[modify] https://crrev.com/8c2764154c196bf55a141141914255ff866b1c12/third_party/WebKit/Source/platform/exported/Platform.cpp
[modify] https://crrev.com/8c2764154c196bf55a141141914255ff866b1c12/third_party/WebKit/public/platform/Platform.h

Apologies, applied the wrong component in bulk.

Setting Internals>Services>Storage to all children of issue 611935