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WebUI preload JS handling is only done on main frame, so it
makes more sense to be moved from ChromeRenderViewObserver to
ChromeRenderFrameObserver where it can also access binding
registry.

Using Associated Interface since tests were failing / flaky
without it, showing errors such as:
* encountered javascript console error(s):
* Uncaught ReferenceError: runTest is not defined
Calls to this mojo method that preloads javascript libraries must
be executed prior to the call to run JS tests.

Bug: 577685
Change-Id: I431158867f60eb40824cbdf9db7d5d5681cbfe5b
Reviewed-on: https://chromium-review.googlesource.com/595019Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491617}