Adding the assert() that checks for the correct view source highlighting

This assert was disabled because it got triggered upon incorrect view-source highlighting.This happened because characters for the end tags were getting consumed by the tokenizer at the same time as it consumed the text of the script tag. 

Bug : https://bugs.webkit.org/show_bug.cgi?id=62971.
Since it is fixed i think we can enable the assert.

Review URL: https://codereview.chromium.org/260033002

git-svn-id: svn://svn.chromium.org/blink/trunk@172778 bbb929c8-8fbe-4397-9dbb-9b2b20218538