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When a button's textContent changes, post a textChanged notification.
A button's textContent is a child of the button container, but in the
accessibility tree button elements cannot have children, so
ChildrenChanged previously did not update the button.

Bug: 793078
Test: DumpAccessibilityTreeTest.AccessibilityButtonContentChanged
Change-Id: I2be389c77763cb60fd2fc9951070370856b0c894
Reviewed-on: https://chromium-review.googlesource.com/1110658
Commit-Queue: James Wallace-Lee <jamwalla@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570786}