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This CL only affects the behavior of Select-to-Speak when STS is
triggered by selection followed by Search-S (it does not affect STS
triggered from the tray or by mouse selection). Since keyboard selection
does not highlight any nodes marked user-select:none, STS follows the
visual behavior here by ignoring these nodes when spoken out as well.
While this causes the behavior to be inconsistent between mouse- and
keyboard-based invocation, it follows the user's mental model: when a
rectangular selection is made with the mouse, STS speaks out everything
within the rectangle that the user draws, whereas when a selection is
made with the cursor and STS invoked from the keyboard, STS speaks out
whatever was actually selected.

Also added a browser test to verify this behavior.

Bug: 830106
Change-Id: I8552e7bda40f150df2ab4bb0338de706a717f30e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426973
Commit-Queue: Ajit Narayanan <ajitnarayanan@google.com>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810768}