build: Generalize the |proprietary_codecs| check from 7db2189.
build: Generalize the |proprietary_codecs| check from 7db2189.

Commit 7db2189 ("Fix the video play issue after M51 rebase") correctly
mentioned the reason for moving the `proprietary_codecs` assignment to a
`conditions` block, but needlessly checked for the `OS` variable instead
of just adding a check that is always true to better simulate what we
were doing before (an unconditional assignment).

Since gyp's evaluation rules are very confusing, also add more comments
to `common.gypi` explaining what is really going on.

BUG=XWALK-7032

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