Also, the UseCounter::Count should take blink::WebLocalFrame or blink::WebDocument instead of ExecutionContext.

talking of WebFrame and WebDocument suggests you're talking about content/renderer and content/child, not content/browser? Which also means you're talking about layers of code that ultimately should hopefully disappear entirely or almost entirely. But I guess if you have a need for it now it might still be worth adding....

Yes, it's in /content/renderer/media/webrtc.

Where is a work-around now and it's a little hacky (origin trial is queried by third_party/blink/renderer/modules/peerconnection/rtc_peer_connection.cc, which exposes the result). It would made sense to implement new origin-trials API for WebFrame in a long run. But if there's some big change coming soon, then we can leave it as is. 

Oh, and re #2, not sure why you would need UseCounter::Count to take a WebLocalFrame? You can already just call WebLocalFrame::BlinkFeatureUsageReport for use counting from content.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3697ff39b0ff0b31ae68f8040b8c1f5d786d1467

commit 3697ff39b0ff0b31ae68f8040b8c1f5d786d1467
Author: Ian Clelland <iclelland@chromium.org>
Date: Thu Jun 07 16:03:43 2018

Allow origin trials to be queried from /content/renderer

This adds a WebOriginTrials interface, which can be used to query the state of
an origin trial given a WebDocument.

Bug:  845995 
Change-Id: Ifbaed167bcfd2588436bd32f6f03d3f57171434c
Reviewed-on: https://chromium-review.googlesource.com/1088310
Commit-Queue: Ian Clelland <iclelland@chromium.org>
Reviewed-by: Jason Chase <chasej@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565283}
[modify] https://crrev.com/3697ff39b0ff0b31ae68f8040b8c1f5d786d1467/content/renderer/render_view_browsertest.cc
[add] https://crrev.com/3697ff39b0ff0b31ae68f8040b8c1f5d786d1467/third_party/blink/public/web/web_origin_trials.h
[add] https://crrev.com/3697ff39b0ff0b31ae68f8040b8c1f5d786d1467/third_party/blink/renderer/build/scripts/make_web_origin_trials.py
[add] https://crrev.com/3697ff39b0ff0b31ae68f8040b8c1f5d786d1467/third_party/blink/renderer/build/scripts/templates/web_origin_trials.cc.tmpl
[modify] https://crrev.com/3697ff39b0ff0b31ae68f8040b8c1f5d786d1467/third_party/blink/renderer/core/BUILD.gn
[modify] https://crrev.com/3697ff39b0ff0b31ae68f8040b8c1f5d786d1467/third_party/blink/renderer/core/exported/web_document_test.cc
[add] https://crrev.com/3697ff39b0ff0b31ae68f8040b8c1f5d786d1467/third_party/blink/renderer/core/testing/data/origin-trial-dummy.html