Add measurement command line args to SampleApp

This way, we may be able to use SampleApp as a performance benchmark
tool that's closer to real-world use cases than nanobench.

For example, we can now run

./out/Release/SampleApp --slide Chart --backendTiles 8 --measureMS 2000

to test the threaded backend.

Bug: skia:
Change-Id: I92b177624bc8d16c5b4d3ad122319882e8783101
Reviewed-on: https://skia-review.googlesource.com/50801
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Yuqian Li <liyuqian@google.com>
2 files changed