Refactor SourceBufferStream::InsertIntoExistingRange().

Splits InsertIntoExistingRange() into IsNextTimestampValid() and
PrepareRangesForNextAppend() so that it is clear that these two
operations are independent. Separating out the PrepareRangesForNextAppend()
stage from the actual append will also make it easier to fix situations
where stale range_for_next_append_ values are being used.

BUG=303413
TEST=No change in functionality. All tests still pass.

Review URL: https://codereview.chromium.org/38703002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230645 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230645