Clean up GestureEventDetails' constructors.

In one of the GestureEventDetails' constructors, the arguments
delta_x, delta_y sometime are not used, so we should make a new
constructor which only takes one argument EventType.

BUG=350942

Review URL: https://codereview.chromium.org/565583005

Cr-Commit-Position: refs/heads/master@{#294760}