DevTools: add support for polling for coverage data in CSS agent

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

 {
     "version": { "major": "1", "minor": "2" },
     "domains": [{
         "domain": "Inspector",
         "experimental": true,
         "types": [],
         "commands": [
             {
                 "name": "enable",
                 "description": "Enables inspector domain notifications."
@@ skipping 2773 matching lines @@
             },
             {
                 "id": "RuleUsage",
                 "type": "object",
                 "properties": [
                     { "name": "styleSheetId", "$ref": "StyleSheetId", "description": "The css style sheet identifier (absent for user agent stylesheet and user-specified stylesheet rules) this rule came from." },
                     { "name": "startOffset", "type": "number", "description": "Offset of the start of the rule (including selector) from the beginning of the stylesheet." },
                     { "name": "endOffset", "type": "number", "description": "Offset of the end of the rule body from the beginning of the stylesheet." },
                     { "name": "used", "type": "boolean", "description": "Indicates whether the rule was actually used by some element in the page." }
                 ],
-                "description": "CSS rule usage information.",
+                "description": "CSS coverage information.",
                 "experimental": true
             },
             {
                 "id": "SourceRange",
                 "type": "object",
                 "properties": [
                     { "name": "startLine", "type": "integer", "description": "Start line of range." },
                     { "name": "startColumn", "type": "integer", "description": "Start column of range (inclusive)." },
                     { "name": "endLine", "type": "integer", "description": "End line of range" },
                     { "name": "endColumn", "type": "integer", "description": "End column of range (exclusive)." }
@@ skipping 351 matching lines @@
                 "name": "getLayoutTreeAndStyles",
                 "parameters": [
                     { "name": "computedStyleWhitelist", "type": "array", "items": { "type": "string" }, "description": "Whitelist of computed styles to return." }
                 ],
                 "returns": [
                     { "name": "layoutTreeNodes", "type": "array", "items": { "$ref": "LayoutTreeNode" } },
                     { "name": "computedStyles", "type": "array", "items": { "$ref": "ComputedStyle" } }
                 ],
                 "description": "For the main document and any content documents, return the LayoutTreeNodes and a whitelisted subset of the computed style. It only returns pushed nodes, on way to pull all nodes is to call DOM.getDocument with a depth of -1.",
                 "experimental": true
-            },
+            },
             {
                 "name": "startRuleUsageTracking",
                 "description": "Enables the selector recording.",
                 "experimental": true
             },
             {
+                "name": "takeCoverageDelta",
+                "description": "Obtain list of rules that became used since last call to this method (or since start of coverage instrumentation)",
+                "returns": [
+                    { "name": "coverage", "type": "array", "items": { "$ref": "RuleUsage" } }
+                ],
+                "experimental": true
+            },
+            {
                 "name": "stopRuleUsageTracking",
                 "returns": [
                     { "name": "ruleUsage", "type": "array", "items": { "$ref": "RuleUsage" } }
                 ],
                 "description": "The list of rules with an indication of whether these were used",
                 "experimental": true
             }
         ],
         "events": [
             {
@@ skipping 1363 matching lines @@
                 "name": "accepted",
                 "description": "Informs that port was successfully bound and got a specified connection id.",
                 "parameters": [
                     {"name": "port", "type": "integer", "description": "Port number that was successfully bound." },
                     {"name": "connectionId", "type": "string", "description": "Connection id to be used." }
                 ]
             }
         ]
     }]
 }