Fix all of the AccessibilityIframeTransform* content_browsertests

The flakiness was all due to the main browser window not being consistently
800 x 600. This was affecting any accessibility test that was asserting
anything about the clipped bounding box of a node with a y-coordinate
greater than 600, for example.

The solution is to make the dimensions of things a bit smaller so
tests aren't more than 600 pixels tall, and to tighten up the
expectations so we aren't relying on anything that might flake.

TBR=katie@chromium.org

Bug: 856566
Change-Id: I391a880c476d3570a97aef1732cd5f2c6de422e4
Reviewed-on: https://chromium-review.googlesource.com/1142532
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576397}
11 files changed