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Introduce a new class MIDIAccessInitializer which initializes MIDIAccess.
Previously the initialization code were on MIDIAccess itself, but this CL
decouples it.

This CL changes the various initialization-related resource ownerships.
In this CL MIDIAccessInitializer owns all initialization related resources
and when the initialization is done (no matter whether successfull or not)
it releases such resources.

Unfortunately MIDIAccess owns MIDIAccessInitializer in this CL because
there is no other natural owner of MIDIAccessInitializer.
Hopefully MIDIAccess will stop owning MIDIAccessInitializer when [1] lands.

1. https://codereview.chromium.org/311733004/

BUG=361041

Review URL: https://codereview.chromium.org/311773003

git-svn-id: svn://svn.chromium.org/blink/trunk@175952 bbb929c8-8fbe-4397-9dbb-9b2b20218538