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This CL adds the multipaste option to the textfield's context menu
under CrOS. In detail, this CL does the following jobs:

(1) It creates a class called ViewsTextServicesContextMenuChromeos to
support the text context menu under CrOS.

(2) Because ViewsTextServicesContextMenuChromeos lives under ui/views,
it cannot access the Ash side functions such as those related to the
multipaste menu directly. To solve such a problem, a new class called
ViewsTextServicesContextMenuImpl is created under ash/public/cpp
and the function to build this new class is injected into the ui/views
side from the ash side.

Bug: 1157669
Change-Id: Ia5a26ab3e89637c4e55a4d8a86235ae2c21d3075
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2595203
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841345}