Fix lifetime handling of SVGPropertyTearOffs.

-Replace SVGStaticPropertyWithParentTearOff with SVGMatrixTearOff
since it's only used in that one place. This means we can get rid
of the templatizing and the method pointer.
-Change SVGPropertyTearOff to keep track of it's child tearoffs
and call detachWrapper on its child tearoffs when it's destroyed
or when it's wrapper is detached.
-Have SVGPropertyTearOff hold WeakPtrs to the child tearoffs
to avoid having a cycle.

BUG=288771

Review URL: https://chromiumcodereview.appspot.com/23995018

git-svn-id: svn://svn.chromium.org/blink/trunk@157801 bbb929c8-8fbe-4397-9dbb-9b2b20218538