Add vulkan build to skpbench.

Add vulkan build to skpbench.

This also fixes the name of the tasks because skpbench is always GPU.
There was a config for monitoring the gpu clock instead of the cpu clock,
but this was removed because the gpu clock isn't very useful at the moment.
cpu clock is basically wall-time, i.e. an accurate measurement of the
entire test.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4873

Change-Id: If1c7dd3141e24f79555ee7b3756f09618df54cb4
Reviewed-on: https://skia-review.googlesource.com/4873
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Eric Boren <borenet@google.com>