platform-graphics: Add command to plot frame time to profile-analysis tool

Command plot-frame-time plots the time spent in either the CPU or
the GPU for either or both profiles.

BUG=None
TEST=None

Change-Id: Ieb9ba632d9811cbf63991bb4d602e5aa3d92cdf3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/graphics/+/2125245
Tested-by: Georges Winkenbach <gwink@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>
Reviewed-by: Ilja H. Friedel <ihf@chromium.org>