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This feature splits the scroll to text functionality off of the Shared
Highlighting flag. This is needed because:

* The Scroll-to-text portion will be launching first
* The Scroll-to-text portion is being built in //ios/web to enable
  external usage.

Bug: 1099268
Change-Id: I60cbe5d20907d4889aa46a051ec335dfe55b59b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2329947Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Tommy Martino <tmartino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793675}