Fix bogus math in object allocation.

When a size_t is convert from a very large number to ptrdiff_t, it
becomes negative causing the existing block to be used instead of
allocating a new block.

BUG=chromium:744109

Change-Id: I0bf98e3fb924851c162f6eca43d29a3f40dc9eaa
Reviewed-on: https://skia-review.googlesource.com/34541
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed