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This functionality is based on the Selection API draft specification and is already supported in Blink by other types of user initiated selection actions:
https://w3c.github.io/selection-api/#selectstart-event

Tests are in the following patch:
https://chromium-review.googlesource.com/c/chromium/src/+/1350195/

R=dmazzoni@chromium.org

Bug: 639340
Change-Id: I5a7793c71b02f2e92fafb7fb9731b6ebf288a412
Tested: with unit tests in another patch
Reviewed-on: https://chromium-review.googlesource.com/c/1347214
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611400}