DOMSnapshot.getSnapshot to expose the document character encoding format

Bug: 
Change-Id: I44f80c51d5e96816bf937eb46f000c0fdcfe271f
Reviewed-on: https://chromium-review.googlesource.com/591670
Commit-Queue: Alex Clarke <alexclarke@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490736}

headless/lib/dom_tree_extraction_expected_nodes.txt

@@ -8,6 +8,7 @@
       "y": 0.0
    },
    "childNodeIndexes": [ 1 ],
+   "documentEncoding": "windows-1252",
    "documentURL": "http://127.0.0.1/dom_tree_test.html",
    "layoutNodeIndex": 0,
    "nodeName": "#document",
@@ -222,6 +223,7 @@
       "y": 0.0
    },
    "childNodeIndexes": [ 19 ],
+   "documentEncoding": "windows-1252",
    "documentURL": "http://127.0.0.1/iframe.html",
    "layoutNodeIndex": 7,
    "nodeName": "#document",

third_party/WebKit/LayoutTests/inspector-protocol/dom-snapshot/dom-snapshot-getSnapshot-expected.txt

@@ -12,7 +12,8 @@ Tests DOMSnapshot.getSnapshot method.
       "layoutNodeIndex": 0,
       "documentURL": "<string>",
       "baseURL": "<string>",
-      "contentLanguage": "de"
+      "contentLanguage": "de",
+      "documentEncoding": "windows-1252"
     },
     {
       "nodeType": 1,
@@ -361,7 +362,8 @@ Tests DOMSnapshot.getSnapshot method.
       ],
       "layoutNodeIndex": 19,
       "documentURL": "<string>",
-      "baseURL": "<string>"
+      "baseURL": "<string>",
+      "documentEncoding": "windows-1252"
     },
     {
       "nodeType": 1,

third_party/WebKit/LayoutTests/inspector-protocol/dom-snapshot/dom-snapshot-getSnapshot-input-value-expected.txt

@@ -11,7 +11,8 @@ Tests DOMSnapshot.getSnapshot method returning input values.
       ],
       "layoutNodeIndex": 0,
       "documentURL": "<string>",
-      "baseURL": "<string>"
+      "baseURL": "<string>",
+      "documentEncoding": "windows-1252"
     },
     {
       "nodeType": 1,

third_party/WebKit/Source/core/inspector/InspectorDOMSnapshotAgent.cpp

@@ -246,6 +246,8 @@ int InspectorDOMSnapshotAgent::VisitNode(Node* node) {
     value->setBaseURL(InspectorDOMAgent::DocumentBaseURLString(document));
     if (document->ContentLanguage())
       value->setContentLanguage(document->ContentLanguage().Utf8().data());
+    if (document->EncodingName())
+      value->setDocumentEncoding(document->EncodingName().Utf8().data());
   } else if (node->IsDocumentTypeNode()) {
     DocumentType* doc_type = ToDocumentType(node);
     value->setPublicId(doc_type->publicId());

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

@@ -3432,6 +3432,7 @@
                     { "name": "documentURL", "type": "string", "optional": true, "description": "Document URL that <code>Document</code> or <code>FrameOwner</code> node points to." },
                     { "name": "baseURL", "type": "string", "optional": true, "description": "Base URL that <code>Document</code> or <code>FrameOwner</code> node uses for URL completion." },
                     { "name": "contentLanguage", "type": "string", "optional": true, "description": "Only set for documents, contains the document's content language." },
+                    { "name": "documentEncoding", "type": "string", "optional": true, "description": "Only set for documents, contains the document's character set encoding." },
                     { "name": "publicId", "type": "string", "optional": true, "description": "<code>DocumentType</code> node's publicId." },
                     { "name": "systemId", "type": "string", "optional": true, "description": "<code>DocumentType</code> node's systemId." },
                     { "name": "frameId", "$ref": "Page.FrameId", "optional": true, "description": "Frame ID for frame owner elements." },