Add cumulative fps to SampleApp

Add cumulative fps to SampleApp

In some samples (whose frame time is really really small),
the fps number jumps so fast that I can hardly see even
the first digit of the number. This problem will become
more severe for threaded backend which substantially lowers
the frame time. Taking an average over  a long time would
give me a much more stable fps number.

Bug: skia:
Change-Id: Ie6052b4735d9410d5e644331bf025b5bf9f40323
Reviewed-on: https://skia-review.googlesource.com/17823
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>