Revert of SegmentedString::push() should always push a char in front

Revert of SegmentedString::push() should always push a char in front (patchset #2 id:20001 of https://codereview.chromium.org/1308573006/ )

Reason for revert:
HTMLEntityParserTest.ConsumeHTMLEntityIncomplete fails on two Win bots

The newly added test is failing on WebKit Win7 (dbg) and WebKit Win Oilpan
(dbg) with the following message:

[ RUN      ] HTMLEntityParserTest.ConsumeHTMLEntityIncomplete
ASSERTION FAILED: !notEnoughCharacters

Sorry for the revert! Full output @ http://goo.gl/tnYH67

Original issue's description:
> SegmentedString::push() should always push a char in front
> 
> Before this CL, SegmentedString::push() had an exotic behavior, where
> two consecutive push would swap its order.
> 
> This CL changes the push() implementation so that it would always push
> the new char in front of the SegmentedString.
> This CL also updates the HTMLEntityParser code which relied on the behavior.
> 
> BUG=None
> TEST={SegmentedStringTest,HTMLEntityParserTest}.*
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201294

TBR=tasak@google.com,tzik@chromium.org,tkent@chromium.org,yosin@chromium.org,kouhei@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=None

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=201318

[Peter Beverloo, 2015-08-27 15:33:19]

Created Revert of SegmentedString::push() should always push a char in front

[commit-bot: I haz the power, 2015-08-27 15:33:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1311043004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1311043004/1

[commit-bot: I haz the power, 2015-08-27 15:33:54]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=201318