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This button compliments the previously named 'Import clipboard' button. With
this CL, we have 4 buttons to export to or import from the clipboard or a file.

Additionally, this CL renames the buttons and modifies accesskeys for clarity:
- Copy (alt+c)
- Paste (alt+v)
- Download (alt+n)
- Upload (alt+m)
The 'Lock cursor position to end of input' checkbox also has its accesskey
changed from alt+c to alt+x.

Bug: 937321
Change-Id: Ieada086afd7e2d35e7b164679e1087245050ef91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497117Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638248}