Specify an internal ID to ::-webkit-file-upload-button

This CL makes the code robust for UA shadow tree changes.
This CL has no behavior changes.

Bug: 1040828
Change-Id: I373b5c01255149890f25b194f3777199dbaad2a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2011465
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733518}
4 files changed