Rename BrowserAccessibility::GetParent to PlatformGetParent.

In prep of refactoring BrowserAccessibility, we are renaming
BrowserAccessibility::GetParent to BrowserAccessibility::PlatformGetParent
to avoid a conflict with AXPlatformNodeDelegate::GetParent.

BUG=703369

Review-Url: https://codereview.chromium.org/2761853004
Cr-Commit-Position: refs/heads/master@{#459246}
13 files changed