[heap] remove alignment requirement from AllocationInfo::limit

While all allocated objects need to have correct alignment, this requirement
doesn't make a lot of sense for the allocation limit. Removing this assertion
would makes it possible to make the inline-allocation-observers accurate. This
could be done in other ways, but removing this assertion is the most elegant way
to go about it.

R=hpayer@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1444883003

Cr-Commit-Position: refs/heads/master@{#32013}

src/heap/spaces.h

@@ -1489,8 +1489,6 @@ class AllocationInfo {
   Address* top_address() { return &top_; }
 
   INLINE(void set_limit(Address limit)) {
-    SLOW_DCHECK(limit == NULL ||
-                (reinterpret_cast<intptr_t>(limit) & kHeapObjectTagMask) == 0);
     limit_ = limit;
   }