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This was originally needed so that we wouldn't cast a
BrowserAccessibility* into a platform-specific subclass
in a unit test that was explicitly constructing the
parent class.

We got rid of all such unit tests now, so there's no longer a
need for IsNative.

Bug: None
Change-Id: I934f08f22aa497c3894ee60c0c8f13fb046fc98e
Relnotes: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2141047Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757753}