New tabCapture.captureOffscreenTab API, initially for Presentation API 1UA mode

chrome/common/extensions/api/tab_capture.idl

Index: chrome/common/extensions/api/tab_capture.idl
diff --git a/chrome/common/extensions/api/tab_capture.idl b/chrome/common/extensions/api/tab_capture.idl
index c6489d6f6980c167c3b898864692979588bec2d5..6382ebf2173027f0b9a0d12d5d3bdf996d048142 100644
--- a/chrome/common/extensions/api/tab_capture.idl
+++ b/chrome/common/extensions/api/tab_capture.idl
@@ -66,6 +66,33 @@ namespace tabCapture {
     // to prevent redundant requests for the same tab).
     // |callback| : Callback invoked with CaptureInfo[] for captured tabs.
     static void getCapturedTabs(GetCapturedTabsCallback callback);
+
+    // Creates an off-screen tab and navigates it to the given |startUrl|.
+    // Then, capture is started and a MediaStream is returned via |callback|.
+    //
+    // Off-screen tabs are isolated from the user's normal browser experience.
+    // They do not show up in the browser window or tab strip, nor are they
+    // visible to extensions (e.g., via the chrome.tabs.* APIs).
+    //
+    // An off-screen tab remains alive until one of three events occurs: 1. All
+    // MediaStreams providing its captured content are closed; 2. the page
+    // self-closes (e.g., via window.close()); 3. the extension that called
+    // captureOffscreenTab() is unloaded.
+    //
+    // Sandboxing: The off-screen tab does not have any access whatsoever to the
+    // local user profile (including cookies, HTTP auth, etc.).  Instead, it is
+    // provided its own sandboxed profile.  Also, it cannot access any
+    // interactive resources such as keyboard/mouse input, media recording
+    // devices (e.g., web cams), copy/paste to/from the system clipboard, etc.
+    //
+    // Note: This is a new API, currently only available in Canary/Dev channel,
+    // and may change without notice.
+    //
+    // |options| : Constraints for the capture and returned MediaStream.
+    // |callback| : <code>null</code> on error.
+    static void captureOffscreenTab(DOMString startUrl,
+                                    CaptureOptions options,
+                                    GetTabMediaCallback callback);
   };
 
   interface Events {