Revert of TextRun::length() should return an unsigned

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

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=183970

[jochen (gone - plz use gerrit), 2014-10-20 10:42:56]

Created Revert of TextRun::length() should return an unsigned

[commit-bot: I haz the power, 2014-10-20 10:43:35]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/669633002/1

[commit-bot: I haz the power, 2014-10-20 10:44:54]

Committed patchset #1 (id:1) as 183970