Add missing adoptRef() on SkPicture.
Add missing adoptRef() on SkPicture.

Looks like the adoptRef() has been missing since first written in
https://codereview.chromium.org/844653005

The following two tests should be failing, but aren't:
 - inspector/layers/layer-canvas-log.html
 - inspector-protocol/layers/paint-profiler.html

Those two tests are not currently failing because we changed SkPicture's base
class from SkRefCnt (which in Blink has the debug checks that'd cause those
tests to fail) to SkNVRefCnt<SkPicture>, which is a little more memory
efficient.  I happened to try switching its base class back to SkRefCnt in an
unrelated CL and found this bug.

We switched the base class in November and this code landed in January.  :(

The attached Skia issue tracks that SkNVRefCnt does not have the same debug
checks for adoptionRequired() and adopted().  After landing this I'll try to
switch SkPicture's base class back SkRefCnt; after doing this, the remaining
cleanup is less urgent, as no other public-facing classes use SkNVRefCnt.

BUG=skia:3847

Review URL: https://codereview.chromium.org/1136123011

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@195549 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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