Make mojo buffer allocators more robust.

mojo/public/cpp/bindings/lib/scratch_buffer.h

Index: mojo/public/cpp/bindings/lib/scratch_buffer.h
diff --git a/mojo/public/cpp/bindings/lib/scratch_buffer.h b/mojo/public/cpp/bindings/lib/scratch_buffer.h
index 462c6febc1300134dca077a10f3af08ace18d8bb..64dabce3502477d1e9b8b3a0343918160e2c8cec 100644
--- a/mojo/public/cpp/bindings/lib/scratch_buffer.h
+++ b/mojo/public/cpp/bindings/lib/scratch_buffer.h
@@ -24,7 +24,10 @@ class ScratchBuffer : public Buffer {
       MOJO_OVERRIDE;
 
  private:
-  enum { kMinSegmentSize = 512 };
+  enum {
+    kMinSegmentSize = 512,
+    kMaxSegmentSize = 1024 * 1024 * 1024,
+  };
 
   struct Segment {
     Segment* next;
@@ -33,7 +36,7 @@ class ScratchBuffer : public Buffer {
   };
 
   void* AllocateInSegment(Segment* segment, size_t num_bytes);
-  void AddOverflowSegment(size_t delta);
+  bool AddOverflowSegment(size_t delta);
 
   char fixed_data_[kMinSegmentSize];
   Segment fixed_;