Move implementation of Mojo method GetTextSurroundingSelection() to Blink

Move this method from the content.mojom.Frame Mojo interface into Blink,
as part of a new blink.mojom.SurroundingText Mojo interface, and add a new
class there to implement such interface (i.e. blink::SurroundingTextImpl),
to replace usages of content.mojom.Frame's GetTextSurroundingSelection()
from the browser process.

Note that this SurroundingTextImpl class still relies on WebSurroundingText
and WebLocalFrame to keep the change smaller, but this is a temporary step
that will be corrected once WebSurroundingText has been moved out of the
public API and into renderer/core, which will happen on a follow-up CL.

Bug: 980151
Change-Id: I62fb2d5b24a98773c1acddbe1c58217c1a2d53e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1696968
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679981}
15 files changed