Run constraints processing for screen capture on the main thread.

Some code paths require access to Blink strings that can only be
 accessed on the main thread.
A test that triggers the bug is coming up on crrev.com/c/738040.

Bug: 791992
Change-Id: I55ae617ae623766d4bd975a19070bde410d8af1d
Reviewed-on: https://chromium-review.googlesource.com/809005Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521708}

content/renderer/media/user_media_processor.cc

@@ -458,14 +458,7 @@ void UserMediaProcessor::SetupVideoInput() {
       GetUserMediaRequestFailed(result, failed_constraint_name);
       return;
     }
-    base::PostTaskAndReplyWithResult(
-        worker_task_runner_.get(), FROM_HERE,
-        base::Bind(&SelectSettingsVideoContentCapture,
-                   current_request_info_->web_request().VideoConstraints(),
-                   video_controls.stream_source),
-        base::Bind(&UserMediaProcessor::FinalizeSelectVideoContentSettings,
-                   weak_factory_.GetWeakPtr(),
-                   current_request_info_->web_request()));
+    SelectVideoContentSettings();
   }
 }
 
@@ -528,13 +521,12 @@ void UserMediaProcessor::FinalizeSelectVideoDeviceSettings(
   GenerateStreamForCurrentRequestInfo();
 }
 
-void UserMediaProcessor::FinalizeSelectVideoContentSettings(
-    const blink::WebUserMediaRequest& web_request,
-    const VideoCaptureSettings& settings) {
+void UserMediaProcessor::SelectVideoContentSettings() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
-  if (!IsCurrentRequestInfo(web_request))
-    return;
-
+  DCHECK(current_request_info_);
+  VideoCaptureSettings settings = SelectSettingsVideoContentCapture(
+      current_request_info_->web_request().VideoConstraints(),
+      current_request_info_->stream_controls()->video.stream_source);
   if (!settings.HasValue()) {
     blink::WebString failed_constraint_name =
         blink::WebString::FromASCII(settings.failed_constraint_name());

content/renderer/media/user_media_processor.h

@@ -7,6 +7,7 @@
 
 #include <memory>
 #include <string>
+#include <utility>
 #include <vector>
 
 #include "base/callback_forward.h"
@@ -260,9 +261,7 @@ class CONTENT_EXPORT UserMediaProcessor
   void FinalizeSelectVideoDeviceSettings(
       const blink::WebUserMediaRequest& web_request,
       const VideoCaptureSettings& settings);
-  void FinalizeSelectVideoContentSettings(
-      const blink::WebUserMediaRequest& web_request,
-      const VideoCaptureSettings& settings);
+  void SelectVideoContentSettings();
 
   void GenerateStreamForCurrentRequestInfo();