I have been hacking on supporting audio element in worker context by redefining HTMLMediaElement IDL: https://chromium-review.googlesource.com/c/562568/

I am at a stage where loader attempts to fetch the file url, but the media loader is tied with WebFrame. I have landed a few cleanup patches to untangle the loader from WebFrame:
- https://chromium-review.googlesource.com/c/580671/
- https://chromium-review.googlesource.com/c/581610/
- https://chromium-review.googlesource.com/c/585232/
- https://chromium-review.googlesource.com/c/592152/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7166d4dac3022723d3a9ff5ce163ab22a7871412

commit 7166d4dac3022723d3a9ff5ce163ab22a7871412
Author: Alok Priyadarshi <alokp@chromium.org>
Date: Sat Jul 29 04:04:25 2017

Use unique_ptr instead of linked_ptr for MediaFactory::url_index_.

Bug: 750311
Change-Id: I56480223e0d896660f4c98c477c6a245643e1d1b
Reviewed-on: https://chromium-review.googlesource.com/591698
Reviewed-by: Fredrik Hubinette <hubbe@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Alok Priyadarshi <alokp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490617}
[modify] https://crrev.com/7166d4dac3022723d3a9ff5ce163ab22a7871412/content/renderer/media/media_factory.cc
[modify] https://crrev.com/7166d4dac3022723d3a9ff5ce163ab22a7871412/content/renderer/media/media_factory.h
[modify] https://crrev.com/7166d4dac3022723d3a9ff5ce163ab22a7871412/media/blink/webmediaplayer_impl.cc
[modify] https://crrev.com/7166d4dac3022723d3a9ff5ce163ab22a7871412/media/blink/webmediaplayer_impl.h
[modify] https://crrev.com/7166d4dac3022723d3a9ff5ce163ab22a7871412/media/blink/webmediaplayer_impl_unittest.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/999498de44d566344138f23949dcd8599ad79cf3

commit 999498de44d566344138f23949dcd8599ad79cf3
Author: Alok Priyadarshi <alokp@chromium.org>
Date: Sat Aug 19 05:53:55 2017

Introduces media::ResourceFetchContext.

This allows injecting a custom URL loader into
ResourceMultiBufferDataProvider and also removes the direct dependency
from UrlIndex/UrlData/ResourceMultiBufferDataProvider on WebLocalFrame.

Bug: 750311
Change-Id: I017be4cc839d866b5dcdd3e5fab3131f2927f555
Reviewed-on: https://chromium-review.googlesource.com/587229
Commit-Queue: Alok Priyadarshi <alokp@chromium.org>
Reviewed-by: Fredrik Hubinette <hubbe@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495804}
[modify] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/content/renderer/media/media_factory.cc
[modify] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/content/renderer/media/media_factory.h
[modify] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/media/blink/BUILD.gn
[add] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/media/blink/mock_resource_fetch_context.cc
[add] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/media/blink/mock_resource_fetch_context.h
[modify] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/media/blink/multibuffer_data_source_unittest.cc
[add] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/media/blink/resource_fetch_context.h
[modify] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/media/blink/resource_multibuffer_data_provider.cc
[modify] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/media/blink/resource_multibuffer_data_provider.h
[modify] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/media/blink/resource_multibuffer_data_provider_unittest.cc
[modify] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/media/blink/url_index.cc
[modify] https://crrev.com/999498de44d566344138f23949dcd8599ad79cf3/media/blink/url_index.h

https://chromium-review.googlesource.com/c/chromium/src/+/596489

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c1a13e6d90c44466043832e06d1ac4ec445ee105

commit c1a13e6d90c44466043832e06d1ac4ec445ee105
Author: Alok Priyadarshi <alokp@chromium.org>
Date: Fri Sep 29 05:14:31 2017

Pass WorkerClients into WorkerThreadForTest::StartWithSourceCode.

Also rearranges code so that WorkerThreadForTest can be defined in one
place instead of having to declare FakeWorkerGlobalScope in between. No
functional change is done here. Purley moving the code around.

Split from
https://chromium-review.googlesource.com/c/chromium/src/+/636684

Bug: 750311
Change-Id: Ic79600567401604651ad579542ee5aabbeb5fa5f
Reviewed-on: https://chromium-review.googlesource.com/691301
Commit-Queue: Alok Priyadarshi <alokp@chromium.org>
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505299}
[modify] https://crrev.com/c1a13e6d90c44466043832e06d1ac4ec445ee105/third_party/WebKit/Source/core/workers/WorkerThreadTestHelper.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cb960730c8e434249751a99a969f482354728771

commit cb960730c8e434249751a99a969f482354728771
Author: Alok Priyadarshi <alokp@chromium.org>
Date: Mon Oct 02 07:24:20 2017

Makes ResourceFetcher histograms thread-safe.

ResourceFetcher instances are created on both - main and worker threads.
The static histograms used by ResourceFetcher instances are however
thread unsafe.

Bug: 750311
Change-Id: Ic0805e4d3e602edf7c469a8516a4e2635493b8d8
Reviewed-on: https://chromium-review.googlesource.com/693395
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505552}
[modify] https://crrev.com/cb960730c8e434249751a99a969f482354728771/third_party/WebKit/Source/platform/Histogram.h
[modify] https://crrev.com/cb960730c8e434249751a99a969f482354728771/third_party/WebKit/Source/platform/loader/fetch/ResourceFetcher.cpp