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This cl reduces the search latency in Keyboard Shortcut Viewer (KSV).
1. Set a time delay to process key events in search mode. In this way,
users can keep typing in the search box.
2. Reuse the KeyboardShortcutItemView between TabbedView and the search
results. This reduces the layout time for |shortcut_label_view_| in
KeyboardShortcutItemView.
3. To highight matched search queries, we cannot avoid layouting
|description_label_view_| in KeyboardShortcutItemView.

Bug: 818882
Test: Tested on Eve and Minnie
Change-Id: I4b36e18fe3f72fba4884c0cc02fea24ff4416630
Reviewed-on: https://chromium-review.googlesource.com/951709Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542020}