Web Animations: Remove ReplaceCompositableValue, replace with AnimatableValue.

BUG=338549

Review URL: https://codereview.chromium.org/140973008

git-svn-id: svn://svn.chromium.org/blink/trunk@165906 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Bindings: fix signature of indexed property deleters: unsigned => uint32_t

indexed property deleters are inconsistent with other indexed operations,
in that they have an unsigned parameter, instead of a uint32_t parameter.
This fixes the inconsistency.

TBR=haraken

Review URL: https://codereview.chromium.org/142713004

git-svn-id: svn://svn.chromium.org/blink/trunk@165907 bbb929c8-8fbe-4397-9dbb-9b2b20218538