Allow synthetic Tap gestures without a preceding TapDown

      While TapDown gestures are a typical part of the touch-gesture pipeline,
they are not required and probably even undesirable in the case of
synthetic Tap gestures (e.g., with tap disambiguation). Allow such
synthetic taps through the gesture stream validator.

BUG=414372

Review URL: https://codereview.chromium.org/572853005

Cr-Commit-Position: refs/heads/master@{#295141}