Bug 1789403 - Implement the size() method in our OTSStream subclass. r=RyanVM

Depends on D156858

Differential Revision: https://phabricator.services.mozilla.com/D156859

gfx/thebes/gfxOTSUtils.h

@@ -35,6 +35,8 @@ class gfxOTSExpandingMemoryStream : public ots::OTSStream {
 
   ~gfxOTSExpandingMemoryStream() { mAlloc.Free(mPtr); }
 
+  size_t size() override { return mLimit; }
+
   // Return the buffer, resized to fit its contents (as it may have been
   // over-allocated during growth), and give up ownership of it so the
   // caller becomes responsible to call free() when finished with it.