PartitionAlloc: make partitionReallocGeneric() match realloc() more closely.

- realloc(0, size) should be equivalent to malloc(size).
- realloc(ptr, 0) should be equivalent to free(ptr).

Review URL: https://codereview.chromium.org/143113002

git-svn-id: svn://svn.chromium.org/blink/trunk@165422 bbb929c8-8fbe-4397-9dbb-9b2b20218538