Re-land: Fix loading accessibility tree for child frame that's already loaded.

(Original issue: http://crrev.com/2299673002 - fixed test failures on Mac
by disabling accessibility between subsequent runs.)

Code leftover from the pre-OOPIF days was causing us to exit early from
the RenderAccessibilityImpl constructor for some child frames that were
already loaded. Everything worked fine if accessibility was already enabled
when loading the frame, but if the frame was already loaded and then
accessibility was enabled, this could cause it to fail to create an
accessibility tree.

The code in RenderAccessibilityImpl is no longer needed because now we
have exactly one accessibility tree per frame.

This wasn't caught by tests because we didn't cover both scenarios, we
always enabled accessibility first.

Added two variants of existing tests that load the page first and then
enable accessibility.

BUG=640231
[email protected]

Review-Url: https://codereview.chromium.org/2299283002
Cr-Commit-Position: refs/heads/master@{#416274}
4 files changed