Make Emulation.setVisibleSize experimental again.

BUG=635947

Review-Url: https://codereview.chromium.org/2367863002
Cr-Commit-Position: refs/heads/master@{#420637}

third_party/WebKit/Source/core/inspector/browser_protocol-1.2.json

@@ -757,6 +757,7 @@
             {
                 "name": "setVisibleSize",
                 "description": "Resizes the frame/viewport of the page. Note that this does not affect the frame's container (e.g. browser window). Can be used to produce screenshots of the specified size. Not supported on Android.",
+                "experimental": true,
                 "parameters": [
                     { "name": "width", "type": "integer", "description": "Frame width (DIP)." },
                     { "name": "height", "type": "integer", "description": "Frame height (DIP)." }

third_party/WebKit/Source/core/inspector/browser_protocol.json

@@ -772,6 +772,7 @@
             {
                 "name": "setVisibleSize",
                 "description": "Resizes the frame/viewport of the page. Note that this does not affect the frame's container (e.g. browser window). Can be used to produce screenshots of the specified size. Not supported on Android.",
+                "experimental": true,
                 "parameters": [
                     { "name": "width", "type": "integer", "description": "Frame width (DIP)." },
                     { "name": "height", "type": "integer", "description": "Frame height (DIP)." }