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.

Remove broken test from last try.

TBR=bungeman@google.com
BUG=chromium:744109

Change-Id: Ifc81bb8f416d64f640730c66bce56a8e4c677173
Reviewed-on: https://skia-review.googlesource.com/35080
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed