Allow to move frames out of Unwinder.

This saves a copy for every unwind we do in heapprofd.

Change-Id: Ife7aa1ce68831729d96ccd4c3e54ca531f98b6bc
diff --git a/libunwindstack/include/unwindstack/Unwinder.h b/libunwindstack/include/unwindstack/Unwinder.h
index ab239c1..dff043b 100644
--- a/libunwindstack/include/unwindstack/Unwinder.h
+++ b/libunwindstack/include/unwindstack/Unwinder.h
@@ -81,6 +81,12 @@
 
   const std::vector<FrameData>& frames() { return frames_; }
 
+  std::vector<FrameData> ConsumeFrames() {
+    std::vector<FrameData> frames = std::move(frames_);
+    frames_.clear();
+    return frames;
+  }
+
   std::string FormatFrame(size_t frame_num);
   static std::string FormatFrame(const FrameData& frame, bool is32bit);