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This CL adds strings and buttons to the signin interception WebUI bubble

Screenshot with this CL:
https://drive.google.com/file/d/1dpWiA-o3ktL5eRaYqMt2kMd2h7IC09t_/view?usp=sharing

Mock:
https://drive.google.com/file/d/134DKn6B_QIX2rJ91kSWS4zpRpm3XUmy5/view?usp=sharing

Bug: 1076880
Change-Id: Ib0d4d2e3768bf2cbcb92beb335de8e87cd6f8c9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2279751
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789423}