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Allows ExtensionApiTest browser_test tests in Chrome to load test
extensions from //extensions/test/data instead of
//chrome/test/data/extensions/api_test.

This lets us share test extensions between different testing binaries,
like browser_tests (in //chrome) and extensions_browsertests
(in //extensions).

The proof-of-concept moves the "messaging/connect_external" test
extension for MessagingApiTest.ConnectExternal and copies a small part
of a secondary extension used in that test, so that Chrome test loads:
  - //extensions/test/data/messaging/connect_external
  - //extensions/test/data/messaging/receiver

Bug: 388893
Change-Id: Ie111fb5a134597a771c58c90ff974109693ade37
Reviewed-on: https://chromium-review.googlesource.com/588251
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491911}