CopyOnWriteBuffer::SetSize to smaller size memcpy less.

webrtc/base/copyonwritebuffer.h

Index: webrtc/base/copyonwritebuffer.h
diff --git a/webrtc/base/copyonwritebuffer.h b/webrtc/base/copyonwritebuffer.h
index 72fb4eea53495a2575ee435efade9d182afd4a69..ecaca026ea4d00fd7ef7438444dd409293b79e19 100644
--- a/webrtc/base/copyonwritebuffer.h
+++ b/webrtc/base/copyonwritebuffer.h
@@ -222,7 +222,12 @@ class CopyOnWriteBuffer {
       return;
     }
 
-    CloneDataIfReferenced(std::max(buffer_->capacity(), size));
+    // Clone data if referenced.
+    if (!buffer_->HasOneRef()) {
+      buffer_ = new RefCountedObject<Buffer>(buffer_->data(),
+        std::min(buffer_->size(), size),
+        std::max(buffer_->capacity(), size));

[kwiberg-webrtc, 2016/09/09 08:28:29]

You maintain the capacity of the original buffer here. Is that supposed to
happen everywhere? (Because it seems to be done in many but not all places. And
it does cost some, so we shouldn't be doing it unless we intend to.)

[danilchap, 2016/09/09 11:25:33]

Yes. there is no need to reallocate with full capacity after this change.
Code adjusted to use smaller buffer.

[danilchap, 2016/09/09 12:58:44]

Actually, it is debatable if SetSize allowed to decrease capacity (realized I
need a version that keeps capacity).
Since capacity is a topic for neighbor CL, I'll keep old behavior where capacity
is kept and land just less memcpy change here.

+    }
     buffer_->SetSize(size);
     RTC_DCHECK(IsConsistent());
   }