Revert of Revert of HTMLConstructionSite: avoid n^2 running time for large scripts. (patchset #1 id:1 of https://codereview.chromium.org/555223002/)

Reason for revert:
Not a culprit. Reverted to bring back the original patch.

Original issue's description:
> Revert of HTMLConstructionSite: avoid n^2 running time for large scripts. (patchset #5 id:80001 of https://codereview.chromium.org/494993002/)
> 
> Reason for revert:
> Speculative revert.
> 
> It may break the following blink sheriff bot:
> http://build.chromium.org/p/chromium.webkit/builders/Android%20Tests%20%28dbg%29/builds/21403
> 
> 
> 
> Original issue's description:
> > HTMLConstructionSite: avoid n^2 running time for large scripts.
> > 
> > Every time background parser sends chunk, tree is flushed.
> > 
> > If page contains very large script, then script node content is updated
> > many times. Every update is causes string concatenation.
> > 
> > Solution: do not flush pending text until it is mandatory.
> > 
> > Test: https://codereview.chromium.org/500363002
> > Test depends on: https://codereview.chromium.org/544453004/
> > 
> > BUG=410790
> > 
> > Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=181635
> 
> TBR=eseidel@chromium.org,kouhei@chromium.org,eustas@chromium.org
> NOTREECHECKS=true
> NOTRY=true
> BUG=410790
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=181664

TBR=eseidel@chromium.org,kouhei@chromium.org,eustas@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=410790

Review URL: https://codereview.chromium.org/558743002

git-svn-id: svn://svn.chromium.org/blink/trunk@181668 bbb929c8-8fbe-4397-9dbb-9b2b20218538