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

Review URL: https://codereview.chromium.org/494993002

git-svn-id: svn://svn.chromium.org/blink/trunk@181635 bbb929c8-8fbe-4397-9dbb-9b2b20218538