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This CL implements the feature that the selected item is able to be
deleted by pressing the backspace key. If no item is selected, pressing
the backspace key does nothing.

Bug: 1114773
Change-Id: I179050d4c15b9ef93b3b4da2e9bbc8395bf3a0fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390859Reviewed-by: David Black <dmblack@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805528}