Rework inspector-protocol/input/dispatchMouseEvent test.

- Added utility function InspectorTest.executeInPage.
- Postpone event listening to skip noise events.
- Rebaseline for Windows, as we handle contextmenu on mouseup there (see WebViewImpl::handleMouseUp).
- Rebaseline for Linux, or it will pick up Windows expectation.

BUG=243596

Review URL: https://codereview.chromium.org/278313002

git-svn-id: svn://svn.chromium.org/blink/trunk@173958 bbb929c8-8fbe-4397-9dbb-9b2b20218538