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This CL
1. Maps std:x|y URLs to std:x (according to the draft spec [1]),
2. Then maps to std-internal://x/index.js (not spec'ed [2])
   in GetInternalURL(),
3. Then retrieves and serves the script contents for std-internal:
   URLs in GetSourceText() using ui::ResourceBundle()
   from blink_resources.pak.

The source JavaScript files for Layered APIs are placed under
third_party/blink/renderer/core/script/resources/layered_api
and bundled into blink_resources.pak.

This CL introduces std: and std-internal: schemes that are used in Blink side
(std: is user-facing while std-internal: is mostly invisible for users)
but not outside Blink.

[1] Spec:
https://github.com/drufball/layered-apis/blob/master/spec.md
[2] Implementation design doc:
https://docs.google.com/document/d/1V-WaCZQbBcQJRSYSYBb8Y6p0DOdDpiNDSmD41ui_73s/edit

Bug: 829084
Change-Id: I61fc0a2f17d937db6e859303ab8a84e4b2b87850
Reviewed-on: https://chromium-review.googlesource.com/1013321
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555524}