We would like to share the event handling logic of RenderWidgetHostViewAura with RenderWidgetHostViewMus. As a first step this change refactors the EventHandler code out of RenderWidgetHostViewAura into its own class

TEST=manually run browser, dry-run of CQ to verify all existing tests.
BUG=631270

Review-Url: https://chromiumcodereview.appspot.com/2317333002
Cr-Commit-Position: refs/heads/master@{#426815}
10 files changed