Initialize the pad memory in GrBufferAllocPool.

src/gpu/GrBufferAllocPool.cpp

Index: src/gpu/GrBufferAllocPool.cpp
diff --git a/src/gpu/GrBufferAllocPool.cpp b/src/gpu/GrBufferAllocPool.cpp
index 26a1d1583725ee120b33d59797d92f926fae824e..f23311877bd4c8387e21fc31f13ea56ad54ec488 100644
--- a/src/gpu/GrBufferAllocPool.cpp
+++ b/src/gpu/GrBufferAllocPool.cpp
@@ -184,6 +184,7 @@ void* GrBufferAllocPool::makeSpace(size_t size,
         size_t pad = GrSizeAlignUpPad(usedBytes,
                                       alignment);
         if ((size + pad) <= back.fBytesFree) {
+            memset((void*)(reinterpret_cast<intptr_t>(fBufferPtr) + usedBytes), 0, pad);
             usedBytes += pad;
             *offset = usedBytes;
             *buffer = back.fBuffer;