Change name of files and class in agc/histogram* in order to avoid issue file-name clash in build files

The changes are done in several patches in order to make
the review easier.

NOTRY=True
BUG=webrtc:5949

Review-Url: https://codereview.webrtc.org/2051443002
Cr-Original-Commit-Position: refs/heads/master@{#13068}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: bbe423312d9321e7de3144faba4198072338745a
9 files changed