[getUserMedia] Remove restrictions for mics without audio processing

Prior to this CL, if a mic was opened, it could not be opened again
with different settings, except for echoCancellation when already
opened using a ProcessedLocalAudioSource.

After this CL, some of the restrictions are removed for getUserMedia:
1. If the mic is opened using a ProcessedLocalAudioSource, it can
be opened again with a different ProcessedLocalAudioSource with a
different echoCancellation setting (same as before), but now it can
also be opened with a non-processed source.
2. If the mic is opened with a non-processed source, it can be opened
again with any type of source and settings.

As before, if, after constraints processing, a new track requests the
same settings as an existing source, the existing source is reused.

Note that these changes apply only to getUserMedia. applyConstraints
stays unmodified and still cannot change source properties.

Bug: 1082049
Change-Id: I18f124eca40a9461691a67ac7b2c2d0cde6fb494
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2532078
Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826723}