[DevTools] Add commands for synthetic gestures.
[DevTools] Add commands for synthetic gestures.

This is the second of three CLs to add synthetic gesture commands to the DevTools Input domain:

1. crrev.com/846113004
2. crrev.com/841013002
3. crrev.com/792883007

BUG=448182

Review URL: https://codereview.chromium.org/841013002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@190588 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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