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This fixes resources loading, and allows WebLayer to work with the
standalone WebView APK, which previously would crash with a
Resources$NotFoundException exception.

Adds a run_weblayer_shell_webview build target for testing with
standalone WebView.

Change-Id: If1063efdfbf462ccf36bc54f796d3b934a456274
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1854204Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Clark DuVall <cduvall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706086}