Expose mBufferId for GraphicBuffer

Bug: 118844974
Test: GraphicBuffer_test
Change-Id: I75a58b21e9943ffed9219a37e0d05c6697232bf4
diff --git a/libs/ui/include/ui/GraphicBuffer.h b/libs/ui/include/ui/GraphicBuffer.h
index 81f6cd9..b73ca2b 100644
--- a/libs/ui/include/ui/GraphicBuffer.h
+++ b/libs/ui/include/ui/GraphicBuffer.h
@@ -153,6 +153,7 @@
     uint32_t getLayerCount() const      { return static_cast<uint32_t>(layerCount); }
     Rect getBounds() const              { return Rect(width, height); }
     uint64_t getId() const              { return mId; }
+    int32_t getBufferId() const { return mBufferId; }
 
     uint32_t getGenerationNumber() const { return mGenerationNumber; }
     void setGenerationNumber(uint32_t generation) {
@@ -247,6 +248,12 @@
 
     uint64_t mId;
 
+    // System unique buffer ID. Note that this is different from mId, which is process unique. For
+    // GraphicBuffer backed by BufferHub, the mBufferId is a system unique identifier that stays the
+    // same cross process for the same chunck of underlying memory. Also note that this only applies
+    // to GraphicBuffers that are backed by BufferHub.
+    int32_t mBufferId = -1;
+
     // Stores the generation number of this buffer. If this number does not
     // match the BufferQueue's internal generation number (set through
     // IGBP::setGenerationNumber), attempts to attach the buffer will fail.