DevTools: [SSP] Implement adding new rule in user stylesheet

LayoutTests/inspector-protocol/css/resources/add-rule.css

Index: LayoutTests/inspector-protocol/css/resources/add-rule.css
diff --git a/LayoutTests/inspector-protocol/css/resources/add-rule.css b/LayoutTests/inspector-protocol/css/resources/add-rule.css
new file mode 100644
index 0000000000000000000000000000000000000000..7fc335df81d7a7cf1cd6f097397c66f05d8f5d7a
--- /dev/null
+++ b/LayoutTests/inspector-protocol/css/resources/add-rule.css
@@ -0,0 +1,21 @@
+#test {
+    box-sizing: border-box;
+}
+
+#test {
+    /* resetting some properties */
+    line-height: 1;
+    font-family: "Arial";
+    color: blue;
+    display: flex; /* flex FTW! */
+}
+
+@media (min-width: 1px) {
+    #test {
+        font-size: 200%;
+    }
+
+    #test {
+    }
+}
+