Add UniqueID to GrDrawTargetCaps.

src/gpu/GrDrawTarget.cpp

Index: src/gpu/GrDrawTarget.cpp
diff --git a/src/gpu/GrDrawTarget.cpp b/src/gpu/GrDrawTarget.cpp
index af00ac8db164bc7e0f5289d5862febaec19866ca..e942d7cf315bf233392f26e78d1ef4babae750a5 100644
--- a/src/gpu/GrDrawTarget.cpp
+++ b/src/gpu/GrDrawTarget.cpp
@@ -1147,3 +1147,13 @@ SkString GrDrawTargetCaps::dump() const {
 
     return r;
 }
+
+uint32_t GrDrawTargetCaps::CreateUniqueID() {
+    static int32_t gUniqueID = SK_InvalidUniqueID;
+    uint32_t id;
+    do {
+        id = static_cast<uint32_t>(sk_atomic_inc(&gUniqueID) + 1);
+    } while (id == SK_InvalidUniqueID);
+    return id;
+}
+