vk: Don't handle swipe gestures when in clamshell mode.

In tablet mode, it's possible to swipe up near the shelf to hide the VK,
but this doesn't make sense in clamshell mode as the shelf is different.

Disable this feature for clamshell mode.

Bug: 1203336
Change-Id: I15fe235b57b624b0251998d614b82b8ef30f56bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2891230


Commit-Queue: Darren Shen <shend@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#882418}