Trivial change to bump minimum block size for Buffer pool

TBR=
BUG=skia:

Review URL: https://codereview.chromium.org/1146073004
diff --git a/src/gpu/GrBufferAllocPool.cpp b/src/gpu/GrBufferAllocPool.cpp
index 6492527..be88b32 100644
--- a/src/gpu/GrBufferAllocPool.cpp
+++ b/src/gpu/GrBufferAllocPool.cpp
@@ -23,7 +23,7 @@
 #endif
 
 // page size
-#define GrBufferAllocPool_MIN_BLOCK_SIZE ((size_t)1 << 12)
+#define GrBufferAllocPool_MIN_BLOCK_SIZE ((size_t)1 << 15)
 
 #define UNMAP_BUFFER(block)                                                               \
 do {                                                                                      \