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ExtensionBrowserTests look for paths beginning with "_test_resources" in
extension resources being loaded, and redirect them. The original
motivation for this was to allow us to re-use common utility files
between loaded test extensions.

We want to use this in the PDF extension tests. For this, we want to
load resources that aren't part of the extension itself as if they
were - specifically, for the sake of exercising module code.

Beef up the _test_resources handling by:
- adding support for subclasses to specify their own parent directory
  through ExtensionBrowserTest::GetTestResourcesParentDir(). This allows
  PDF tests to load resources from chrome/test/data/pdf instead of
  chrome/test/data/extensions.
- adding browser test coverage, including the basic case, a component
  extension case (which the PDF extension is), and the customized
  resource parent.

This is needed to unblock the PDF extension's use of modules, which in
turn is needed for the Polymer 2 -> Polymer 3 migration.

Bug: 1005029
Change-Id: I8a998eebf38bb680ffde72f512b962d83fe75989
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1850779
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Kelvin Jiang <kelvinjiang@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706055}