[SVG] Reinstate RenderSVGResourcePattern's inval-during-paint guard.

The refactoring in https://codereview.chromium.org/453653003 dropped the
double-lookup map insertion, but animated images (at least) can still
trigger paint-time layout invals => the map entry may get deleted while
we're building the pattern.

This CL adds back the required double-lookup idiom.

R=fs@opera.com,pdr@chromium.org,schenney@chromium.org
BUG=426757,426882

Review URL: https://codereview.chromium.org/677683003

git-svn-id: svn://svn.chromium.org/blink/trunk@184370 bbb929c8-8fbe-4397-9dbb-9b2b20218538