Rename FrameTestHelpers namepsace to frame_test_helpers

https://google.github.io/styleguide/cppguide.html#Namespace_Names
> Namespace names are all lower-case.

This CL has no behavior changes.

Bug: 889726
Change-Id: I63e4fd34b8bde448da84a6b2f578dc00a69cf33a
Reviewed-on: https://chromium-review.googlesource.com/c/1293067
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Zhuoyu Qian <zhuoyu.qian@samsung.com>
Cr-Commit-Position: refs/heads/master@{#601511}
49 files changed