TextRun::length() should return an unsigned.

Tightened int types to unsigned in TextRun to make it easier to reason about the code and in the process removed some casts and some ASSERTS that are no longer needed.

Also made the getter type consistent with the corresponding member type for for consistency where applicable.

Review URL: https://codereview.chromium.org/652483002

git-svn-id: svn://svn.chromium.org/blink/trunk@183934 bbb929c8-8fbe-4397-9dbb-9b2b20218538