Revert of TextRun::length() should return an unsigned (patchset #2 id:190001 of https://codereview.chromium.org/652483002/)

Reason for revert:
breaks FindInPageControllerTest.FindInPageFormsTextAreas

Original issue's description:
> 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.
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=183934

TBR=eae@chromium.org,mlamouri@chromium.org,l.gombos@samsung.com
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/669633002

git-svn-id: svn://svn.chromium.org/blink/trunk@183970 bbb929c8-8fbe-4397-9dbb-9b2b20218538