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Implements BaseAudioContext as the base class of an AudioContext and
OfflineAudioContext, moving the relevant attributes and methods from
AudioContext to BaseAudioContext.

The changes in the expected results are mostly caused by exception
messages now saying BaseAudioContext instead of AudioContext.

BUG=555608
TEST=no new tests

Review-Url: https://codereview.chromium.org/1865583002
Cr-Commit-Position: refs/heads/master@{#417297}