Support PNG and quality control in chrome.tabs.captureVisibleTab().

chrome/common/extensions/api/extension_api.json

 [
   {
     "namespace": "extension",
     "unprivileged": true,
     "types": [
       {
         "id": "MessageSender",
         "type": "object",
         "description": "An object containing information about the script context that sent a message or request.",
         "properties": {
@@ skipping 961 matching lines @@
         "description": "Captures the visible area of the currently selected tab in the specified window.",
         "parameters": [
           {
             "type": "integer",
             "name": "windowId",
             "minimum": 0,
             "optional": true,
             "description": "The target window. Defaults to the <a href='windows.html#current-window'>current window</a>."
           },
           {
+            "type": "object",
+            "name": "options",
+            "optional": true,
+            "description": "Set parameters of image capture, such as the format of the resulting image.  This parameter was added in version 5.0 .  Earlier versions of chrome will throw an exception unless you omit it.",
+            "properties": {
+              "format": {
+                "type": "string",
+                "optional": true,
+                "enum": ["jpeg", "png"],
+                "description": "The format of the resulting image.  Default is jpeg."
+              },
+              "quality": {
+                "type": "integer", 
+                "name": "quality",
+                "optional": true,
+                "minimum": 0,
+                "maximum": 100,
+                "description": "When format is 'jpeg', controls the quality of the resulting image.  This value is ignored for PNG images.  As quality is decreased, the resulting image will have more visual artifacts, and the number of bytes needed to store it will decrease."
+              }
+            }
+          },
+          {
             "type": "function", "name": "callback", "parameters": [
-              {"type": "string", "name": "dataUrl", "description": "A data URL of a JPEG encoding of the visible area of the captured tab. May be assigned to the 'src' property of an HTML Image element for display."}
+              {"type": "string", "name": "dataUrl", "description": "A data URL which encodes an image of the visible area of the captured tab. May be assigned to the 'src' property of an HTML Image element for display."}
             ]
           }
         ]
       },
       {
         "name": "executeScript",
         "type": "function",
         "description": "Injects JavaScript code into a page. For details, see the <a href='content_scripts.html#pi'>programmatic injection</a> section of the content scripts doc.",
         "parameters": [
           {"type": "integer", "name": "tabId", "optional": true, "description": "The ID of the tab in which to run the script; defaults to the selected tab of the current window."},
@@ skipping 1724 matching lines @@
         "description": "Triggers a paste operation in the specified tab.",
         "parameters": [
           {"type": "integer", "name": "tabId", "minimum": 0},
           {"type": "function", "name": "callback", "optional": true, "parameters": []}
         ]
       }
     ],
     "events": []
   }
 ]