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Adds a method in AccessibilityBrowserTest that loads an HTML page and waits until ready

Instead of having to create an AccessibilityNotificationWaiter at the start of every test and wait on it,
this method makes loading an HTML document easier and removes repeated code from tests.

This patch also switches to using the modern C++ raw string litterals
which have the advantage that quotes and line breaks don't need to be escaped,
and the HTML could be formatted the same way as in an HTML file.

R=aboxhall@chromium.org

Change-Id: Iaf8e59be8eb7b9bf2b0ca90caaa7cb6c3380622a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696423
Auto-Submit: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Alice Boxhall <aboxhall@chromium.org>
Reviewed-by: Alice Boxhall <aboxhall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676295}