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ScriptExecutor is used to drive programmatic script execution by
extensions into different web frames. While it is reasonably tested
transitively through end-to-end tests for the tabs.executeScript()
method and friends, we should have more targetted testing for its
functionality as well.

Add a browser test with a simple test case of injecting script into
the main world and returning the result as a starting point.

Bug: None
Change-Id: I9a00a6b1141935d35cf1c0317c0ccda734c991e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2528912
Auto-Submit: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826365}