Upgrade to dlmalloc 2.8.6.

This fixes a bug and enables the use of MORECORE_CONTIGUOUS.

Change-Id: Ia7c5d32bdc46e99b3ecb92ee94d1f702c4385d5d
diff --git a/libc/upstream-dlmalloc/malloc.h b/libc/upstream-dlmalloc/malloc.h
index 7ede698..e52c9e5 100644
--- a/libc/upstream-dlmalloc/malloc.h
+++ b/libc/upstream-dlmalloc/malloc.h
@@ -518,7 +518,7 @@
   p = malloc(n);
   assert(malloc_usable_size(p) >= 256);
 */
-size_t dlmalloc_usable_size(void*);
+size_t dlmalloc_usable_size(const void*);
 
 #if MSPACES
 
@@ -601,9 +601,7 @@
 void** mspace_independent_comalloc(mspace msp, size_t n_elements,
                                    size_t sizes[], void* chunks[]);
 size_t mspace_bulk_free(mspace msp, void**, size_t n_elements);
-// BEGIN android-changed: added const
 size_t mspace_usable_size(const void* mem);
-// END android-changed
 void mspace_malloc_stats(mspace msp);
 int mspace_trim(mspace msp, size_t pad);
 size_t mspace_footprint(mspace msp);