[devtools] Add DOMSnapshot domain for dom+layout+style snapshots.

headless/public/headless_devtools_client.h

Index: headless/public/headless_devtools_client.h
diff --git a/headless/public/headless_devtools_client.h b/headless/public/headless_devtools_client.h
index 1b8073e81b54d3a686e04502f526be5a08f29da5..b96f5cadb6e6b97c0cec8f0ffce3e13b7c839f5c 100644
--- a/headless/public/headless_devtools_client.h
+++ b/headless/public/headless_devtools_client.h
@@ -48,6 +48,9 @@ class Domain;
 namespace dom_debugger {
 class Domain;
 }
+namespace dom_snapshot {
+class Domain;
+}
 namespace dom_storage {
 class Domain;
 }
@@ -127,6 +130,7 @@ class HEADLESS_EXPORT HeadlessDevToolsClient {
   virtual device_orientation::Domain* GetDeviceOrientation() = 0;
   virtual dom::Domain* GetDOM() = 0;
   virtual dom_debugger::Domain* GetDOMDebugger() = 0;
+  virtual dom_snapshot::Domain* GetDOMSnapshot() = 0;
   virtual dom_storage::Domain* GetDOMStorage() = 0;
   virtual emulation::Domain* GetEmulation() = 0;
   virtual heap_profiler::Domain* GetHeapProfiler() = 0;