[DevTools] Make screencast gestures follow touch emulation.

Unfortunately, we cannot inject touches and get gestures, so the only
solution is to issue gesture events directly.

What changed:
- left mouse button drag with shift issues pinch (instead of right button);
- mousewheel does not disturb current scroll and pinch;
- reversed pinch zoom direction;
- fixed scroll delta calculation, which was screwed up by |screenOffsetTop| updates.

BUG=324590

Review URL: https://codereview.chromium.org/259583002

git-svn-id: svn://svn.chromium.org/blink/trunk@172661 bbb929c8-8fbe-4397-9dbb-9b2b20218538