Initial VideoProcessing refactoring.

This CL is the first in a series of CLs to refactor
VideoProcessing(Module) to follow Google C++ style guide and make the
code more readable.

This CL removed inheritance from Module, renames variables and makes
VideoProcessingImpl::PreprocessFrame return a frame pointer if there
is a frame to send, nullptr otherwise. The affected CLs also passes git
cl lint.

BUG=webrtc:5259

Review URL: https://codereview.webrtc.org/1482913003

Cr-Original-Commit-Position: refs/heads/master@{#10907}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a8565425bc6b1d3d7b5a0b0e0a49bc4e57c64f21
22 files changed