Rename AbstractAudioContext to BaseAudioContext

A mostly mechanical change renaming AbstractAudioContext to
BaseAudioContext to match the class names used in the WebAudio spec.
This is a first step in actually implementing the BaseAudioContext
class as required.

The change is done using

for f in *.cpp *.h
do
  sed -i 's/AbstractAudioContext/BaseAudioContext/g' $f
done

And manually updating AudioContext.idl and renaming
AbstractAudioContext.* to BaseAudioContext.*, and ordering the
include files correctly.

BUG=624602
TEST=

Review-Url: https://codereview.chromium.org/2103043007
Cr-Commit-Position: refs/heads/master@{#404234}
75 files changed