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Before r201294, SegmentedString::push() had an exotic behavior where two
consecutive push() results are swapped.  SegmentedString stored those two
chars into m_pushedChar{1,2} to avoid messing up SegmentedSubString.

After r201294, SegmentedString::push() is guaranteed to be used for
ungetc()-like usecase, where it reverts the SegmentedString::advance()
which was issued just before the call.

This CL removes m_pushedChar{1,2} optimization, and replace it with
SegmentedSubstring::push() which simply decrement m_data.string{8,16}Ptr.
In pessimistic case, this may end up prepending wtf::String inside
SegmentedSubstring, but this should be very rare.

BUG=None
TESTS=SegmentedStringTest

Review URL: https://codereview.chromium.org/1319913002

git-svn-id: svn://svn.chromium.org/blink/trunk@201517 bbb929c8-8fbe-4397-9dbb-9b2b20218538