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Teach the <file-safe-media> element more about the loaded state of its
child <webview>: make the <webview> send webview-loaded/cleared events
to its parent <file-safe-media>, and have the parent set that state in
the 'loaded' attribute of the child <webview>.

Add an html scroll test: wait for the <webview> to load its content by
waiting for the 'loaded' attribute. Then call window.scrollBy() on the
<webview> and verify that the <webview> was scrolled.

Minor: files_safe_media_webview_content.js: use document.querySelector
to get the #content element to silence a presubmit warning.

Test: browser_tests --gtest_filter="QuickView*openQuickViewScrollHtml"
Bug: 891150
Change-Id: I7bc10a47ff4e50a16ece910174959596615e6fd7
Reviewed-on: https://chromium-review.googlesource.com/1256399
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595791}