[ios] Snapshot for WebState

ios/web/public/web_state/web_state.h

Index: ios/web/public/web_state/web_state.h
diff --git a/ios/web/public/web_state/web_state.h b/ios/web/public/web_state/web_state.h
index 6974f523942f2691752762d1a997bb86f4d2338b..bb155690fd3395d9ee330a0a580baca3cb86c18d 100644
--- a/ios/web/public/web_state/web_state.h
+++ b/ios/web/public/web_state/web_state.h
@@ -35,6 +35,10 @@ class DictionaryValue;
 class Value;
 }
 
+namespace gfx {
+class Image;
+}
+
 namespace web {
 
 class BrowserState;
@@ -248,6 +252,11 @@ class WebState : public base::SupportsUserData {
   // CreateParams::created_with_opener for more details.
   virtual bool HasOpener() const = 0;
 
+  // Takes a full-resolution snapshot of this WebState.

[marq (ping after 24h), 2017/06/21 11:13:23]

Document when |callback| will be called?

[kkhorimoto, 2017/06/21 20:06:52]

You should also document that this API is asynchronous on iOS 11, but
synchronous for previous versions of iOS.

[edchin, 2017/06/22 07:01:07]

Done.

[edchin, 2017/06/22 07:01:07]

Made this asynchronous in pre-ios-11 versions as well. Used SequencedTaskRunner.

+  virtual void TakeSnapshot(

[Eugene But (OOO till 7-30), 2017/06/21 22:11:29]

Do you want to pass width argument? Not every client will need view's size. 

[edchin, 2017/06/22 07:01:07]

Resizing will take place elsewhere. That place will have the infrastructure to
make operations cancellable. So let's keep web simple and move such 
complexity elsewhere.

[Eugene But (OOO till 7-30), 2017/06/22 19:34:14]

What is the advantage of resizing elsewhere? Changing graphics context size will
not make code complex and WKWebView already supports width argument.

[edchin, 2017/06/23 19:03:10]

Per our offline discussion, I've added a CGSize argument. This improves memory 
and runtime efficiency over taking a full-res, full-size snapshot and then
resizing.

+      const base::Callback<void(const gfx::Image& snapshot)>& callback)

[kkhorimoto, 2017/06/21 20:06:52]

Optional: There's some precedent for typedefing Callbacks in web (e.g.
java_script_dialog_callback.h).  You might want to do the same here to make
formatting a little easier.

[edchin, 2017/06/22 07:01:07]

Done.

+      const = 0;
+
  protected:
   friend class WebStateObserver;
   friend class WebStatePolicyDecider;