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This change implements ITextProvider::GetSelection based on existing
selection data from AXTreeData.

Unit tests were added.

Bug: 928948
Change-Id: Ifa5695077e67b312b9efd23051841a29f8fa8e62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1534142
Commit-Queue: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644841}