Set Canvas paths as volatile.

Preliminary work before we turn on distance field rendering for
small paths (e.g. icons).  Any paths marked volatile will not 
be cached in the distance field path system. Canvas paths are 
temporary, so they should be marked as such.

BUG=417817

Review URL: https://codereview.chromium.org/696443002

git-svn-id: svn://svn.chromium.org/blink/trunk@184796 bbb929c8-8fbe-4397-9dbb-9b2b20218538