Remove some now-unneeded caveats in Element related to inline style wrappers. Also some cleanup.

The reason this cleanup is possible is that the CSSOM wrapper is no longer a dependent pointer of the mutable inline style (it now belongs to the Element directly). This means we can safely clear out/constify the mutable inline style and const-ify it if we want, which also allows creating a sharable element data, and removes the need to clear parent element pointers.

The other cleanup is in preparation for getting rid of ensureCSSStyleDeclaration() from MutableStylePropertySet entirely in a followup CL. After this CL there are no calls to it other than in code unrelated to Element's inline style.

BUG=326664

Review URL: https://codereview.chromium.org/119453004

git-svn-id: svn://svn.chromium.org/blink/trunk@164384 bbb929c8-8fbe-4397-9dbb-9b2b20218538