Add Quick View PDF document load test

Add a Quick View loading test for a PDF document. For loading, wait on
the <webview> |src| then detect the PDF plugin <embed> type as a proxy
for "contentChanged".

Note: there's no need to add any PDF scrolling / backgroundColor tests
since those aspects are handled internally by Chrome's PDF viewer.

Bug: 891150
Change-Id: I8d3eb438f1cca6d2d249966fd107a4075c7b67b7
Reviewed-on: https://chromium-review.googlesource.com/c/1267916
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597499}
2 files changed