DevTools: fix CodeMirror interaction with shortcut handling system

The shortcut handler subsystem assumes that a keydown should not invoke
a shortcut handling logic if there is subsequent "input" event.

The codemirror generally relies on input event from its hidden text area
to handle user input, so at this stage it plays well with our shortcut
handler. But additionally it has its own "shortcut handling" machinery,
aka "custom keymaps". When it executes some action for the shortcut,
the text area might not change, and the input event will not trigger.
This is the reason of the misbehavior, described in the patch.

The patch adds public dismissPendingShortcutAction on shortcut handler
so that it could be reused in CodeMirrorTextEditor.

BUG=406865
R=vsevik

Review URL: https://codereview.chromium.org/514633002

git-svn-id: svn://svn.chromium.org/blink/trunk@180978 bbb929c8-8fbe-4397-9dbb-9b2b20218538