Async Clipboard: Automate writeImage/readImage test

- Automate tests for writeImage/readImage by duplicating
web_tests/external/wpt/clipboard-apis/async-write-image-read-image-manual.https.html
to a directory allowing Chrome-specific permissions-helper.js (and fetching via
http server).
- Create appropriate README/OWNERS file for new directory.
- Update web_tests/clipboard/async-clipboard/README.md with new info regarding
testrunner.
- Update external/wpt writeImage/readImage test with simplification found
through this CL.
- Follow-up work: add `if(testRunner)` to automated tests so that they can be run
manually.

Bug: 896479
Change-Id: I8b2b27f6fcc4abc9e11169c196eee1863d80fd82
Reviewed-on: https://chromium-review.googlesource.com/c/1413023
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624401}