Move media/capture/content/feedback_signal_accumulator{.h,_unittest.cc} to media/base

feedback_signal_accumulator.h contains a generic templatised class used
by cast/sender/vp8_encoder.cc and capture/content/video_capture_oracle.cc
Since media/capture is now a component (and so is media/cast), it
makes sense to move it to a common place, media/base being a good
option.

gn and gyp files are updated, and the _unittest.cc file is moved as
well.

This cleanup came up while working on https://crbug.com/626125

BUG=618718
TEST= no new code: all builds and unittests working as before.

Review-Url: https://codereview.chromium.org/2149793003
Cr-Commit-Position: refs/heads/master@{#405389}
8 files changed