skpbench: support pixel phone

Adds a script for pixel hardware with conservatively low clocks.

Bug: skia:
Change-Id: I1ade703ab9f0b4aefc9cf630e3d2efb996afd69f
Reviewed-on: https://skia-review.googlesource.com/62343
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
7 files changed