[DevTools] Definitions of capabilities in protocol.

Source/devtools/protocol.json

Index: Source/devtools/protocol.json
diff --git a/Source/devtools/protocol.json b/Source/devtools/protocol.json
index 0e317104b254f3fcb05f368b688b7a1aa9e09bff..0ea7455e901efa3ced3bf93a956123aa20332272 100644
--- a/Source/devtools/protocol.json
+++ b/Source/devtools/protocol.json
@@ -62,6 +62,12 @@
     {
         "domain": "Page",
         "description": "Actions and events related to the inspected page belong to the page domain.",
+        "capabilities": [
+            {
+                "name": "screencast",
+                "description": "Whether screencast is supported."
+            }
+        ],
         "types": [
             {
                 "id": "ResourceType",
@@ -622,6 +628,12 @@
     {
         "domain": "Emulation",
         "description": "This domain emulates different environments for the page.",
+        "capabilities": [
+            {
+                "name": "deviceMetrics",
+                "description": "Whether device metrics emulation is supported."
+            }
+        ],
         "hidden": true,
         "types": [
             {