Avoid touching gfx::Image from multiple thread in devtools page_handler

PageHandler::ScreencastFrameCaptured creates a gfx::Image and uses it on
another sequence. However, the usage causes a data race as gfx::Image has
a non-threadsafe ref count internally, and its copies can't cross sequences. 

Bug: 600237, 725706
Change-Id: I645d529db1b8887b3c67f81a7916cdab0940b1d0
Reviewed-on: https://chromium-review.googlesource.com/513665Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Taiju Tsuiki (tzik) <tzik@google.com>
Cr-Commit-Position: refs/heads/master@{#474952}

content/browser/devtools/protocol/page_handler.cc

@@ -85,6 +85,12 @@ std::string EncodeImage(const gfx::Image& image,
   return base_64_data;
 }
 
+std::string EncodeSkBitmap(const SkBitmap& image,
+                           const std::string& format,
+                           int quality) {
+  return EncodeImage(gfx::Image::CreateFrom1xBitmap(image), format, quality);
+}
+
 }  // namespace
 
 PageHandler::PageHandler()
@@ -593,8 +599,8 @@ void PageHandler::ScreencastFrameCaptured(cc::CompositorFrameMetadata metadata,
   }
   base::PostTaskWithTraitsAndReplyWithResult(
       FROM_HERE, {base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN},
-      base::Bind(&EncodeImage, gfx::Image::CreateFrom1xBitmap(bitmap),
-                 screencast_format_, screencast_quality_),
+      base::Bind(&EncodeSkBitmap, bitmap, screencast_format_,
+                 screencast_quality_),
       base::Bind(&PageHandler::ScreencastFrameEncoded,
                  weak_factory_.GetWeakPtr(), base::Passed(&metadata),
                  base::Time::Now()));