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This CL makes "cancel" the initially focused button in PWA installation
confirmation dialogs. This is to prevent accidental installations if the
user holds down enter.
The "install" button is still coloured blue as if it were the default action.

Before: https://bugs.chromium.org/p/chromium/issues/attachment?aid=391193&signed_aid=oHH2Y0KPtmjQ0oX90XEI4A==&inline=1
After: https://bugs.chromium.org/p/chromium/issues/attachment?aid=392968&signed_aid=sk4b6UZkAkZ9U6T9qqwovw==&inline=1

Change-Id: I05ac2d11aa15077395c617e5ba067025de6d98ca
Bug: 957848
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1588070
Commit-Queue: Alan Cutter <alancutter@chromium.org>
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Reviewed-by: Jay Harris <harrisjay@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660688}