Log to the console when a dialog is suppressed or would be.

chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc

Index: chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
diff --git a/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc b/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
index dd52e788c6c634380fed2c5ca9a9966a4076e6e1..88cde64276648e60760ca1d07abb0b12a125bcd8 100644
--- a/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
+++ b/chrome/browser/ui/javascript_dialogs/javascript_dialog_tab_helper.cc
@@ -16,6 +16,7 @@
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "chrome/common/chrome_features.h"
 #include "components/app_modal/javascript_dialog_manager.h"
+#include "content/public/browser/render_frame_host.h"
 
 DEFINE_WEB_CONTENTS_USER_DATA_KEY(JavaScriptDialogTabHelper);
 
@@ -80,13 +81,26 @@ void JavaScriptDialogTabHelper::RunJavaScriptDialog(
     content::WebContents* parent_web_contents =
         WebContentsObserver::web_contents();
 
-    if (!IsWebContentsForemost(parent_web_contents) &&
-        message_type == content::JAVASCRIPT_MESSAGE_TYPE_PROMPT) {
-      // Don't allow "prompt" dialogs to steal the user's focus. TODO(avi):
-      // Eventually, for subsequent phases of http://bit.ly/project-oldspice,
-      // turn off focus stealing for other dialog types.
-      *did_suppress_message = true;
-      return;
+    if (!IsWebContentsForemost(parent_web_contents)) {
+      if (message_type == content::JAVASCRIPT_MESSAGE_TYPE_PROMPT) {
+        // Don't allow "prompt" dialogs to steal the user's focus. TODO(avi):
+        // Eventually, for subsequent phases of http://bit.ly/project-oldspice,
+        // turn off focus stealing for other dialog types.
+        *did_suppress_message = true;
+        alerting_web_contents->GetMainFrame()->AddMessageToConsole(
+            content::CONSOLE_MESSAGE_LEVEL_WARNING,
+            "A window.prompt() dialog generated by this page was suppressed "
+            "because this page is not the active tab of the front window. "
+            "Please do not use dialogs to force browser windows to the front.");

[Rick Byers, 2016/11/10 04:18:39]

Nit: add chromestatus.com feature link for now (we can link to devrel docs,
samples, etc. from there).

[Rick Byers, 2016/11/10 04:18:39]

Is there some way for developers to actually do what you suggest in the last
sentence here?  I.e. does the page visibility API give them the exact
information they need to know when not to prompt?  Or is the only real solution
to not use the prompt API at all?  If there's not great guidance here, then you
might just want to omit that last sentence (maybe it's legitimate to still use
prompt but just handle the empty return?).

[Avi (use Gerrit), 2016/11/10 21:04:30]

What they should do is not use prompt() untriggered by user interaction; if the
user is interacting with the page, it'll be foremost.

Let me try rephrasing.

[Avi (use Gerrit), 2016/11/10 21:04:30]

Done.

[Rick Byers, 2016/11/11 00:43:33]

Sounds good - thanks!

+        return;
+      }
+      // For now, let them off the hook with a warning.
+      alerting_web_contents->GetMainFrame()->AddMessageToConsole(
+          content::CONSOLE_MESSAGE_LEVEL_WARNING,
+          "A JavaScript dialog was generated by this page while this page was "
+          "not the active tab of the front window. Such dialogs will soon be "
+          "suppressed. Please do not use dialogs to force browser windows to "
+          "the front.");

[Rick Byers, 2016/11/10 04:18:39]

Do you have UMA data on how often this occurs?  We're pretty sensitive to
console warning spam - we know it causes warning blindness.

Also are you confident enough in the consensus here that we can say "will soon
be"... or should it be "may"?

Again, should ideally have a link for more details - chromestatus feature for
now should be fine.

[Avi (use Gerrit), 2016/11/10 21:04:30]

No, I don't, unfortunately.

Do you want to not warn in this situation?

[Rick Byers, 2016/11/11 00:43:33]

I guess I'd rather discuss this one separately - ideally on an "intent to
deprecate" thread.  Then we could get agreement on a specific removal plan and
include a date/milestone etc.  Otherwise we know the impact to the warning won't
be as high.

     }
 
     if (dialog_) {