Embed keyboard shortcut bit in WebKeyboardEvent

Before a WebKeyboardEvent is forwarded to the renderer, the content
embedder can tag it as a keyboard shortcut. This shortcut bit is passed
along with the input event to the renderer, allowing the renderer to
suppress follow-up Char events for unhandled RawKeyDown events that are
part of a keyboard shortcut.

However, passing around this keyboard shortcut bit separately is awkward
and pollutes a number of input-related methods. Instead, just embed the
shortcut bit directly in the WebKeyboardEvent.

BUG=302492

Review URL: https://codereview.chromium.org/1391843006

Cr-Commit-Position: refs/heads/master@{#354384}
21 files changed