Revert 190605 "Make RenderObject::style() return a const object"

Revert 190605 "Make RenderObject::style() return a const object"

The change may have caused a bad crasher on Canary.

> Make RenderObject::style() return a const object
> 
> The current code was sloppy and allowed accidental mutation
> of RenderStyle outside of style recalc. While this change
> doesn't fix that class of bugs (as we introduce some
> mutable accessors), it greatly reduces the risk of
> them being introduced by mistake.
> 
> In order to make the change work, 2 fields on RenderStyle had
> to be made mutable (unique and explicitInheritance) because
> they are propagated up the tree during style recalc.
> 
> Also StyleSelectorState had to be changed to store the override
> parent style independently of the regular parent style. That's
> required as RefPtr modifies the style by increasing the refCount
> and we have to hold onto the override as it can be a free style.
> 
> Review URL: https://codereview.chromium.org/889563002

TBR=jchaffraix@chromium.org
BUG=460820
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=190673

[Julien - ping for review, 2015-02-23 17:40:51]

Committed patchset #1 (id:1) manually as r190673.