PartitionAlloc: replace no-op initializations to 0 with asserts.
PartitionAlloc: replace no-op initializations to 0 with asserts.

This resolves a very minor inefficiency.

Review URL: https://codereview.chromium.org/1201653002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@197539 bbb929c8-8fbe-4397-9dbb-9b2b20218538

Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk,Pluto-tv/blink-crosswalk