Remaining changes to build all code in src.git with -Wimplicit-fallthrough.

Fixes a bug where if we received png data and then failed to decode
that, we'd try to decode it as jpeg too unintentionally, cf
https://codereview.chromium.org/2943363003/diff/120001/chrome/renderer/chrome_render_frame_observer.cc

This CL was uploaded by git cl split.

R=thestig@chromium.org

Bug: 177475
Change-Id: I1fbfc4b8b1fa009e10ace632a09619f463305cb6
Reviewed-on: https://chromium-review.googlesource.com/891899
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532872}

chrome/renderer/chrome_render_frame_observer.cc

@@ -242,13 +242,12 @@ void ChromeRenderFrameObserver::RequestThumbnailForContextNode(
       if (gfx::PNGCodec::EncodeBGRASkBitmap(
               bitmap, kDiscardTransparencyForContextMenu, &data)) {
         thumbnail_data.swap(data);
-        break;
       }
+      break;
     case chrome::mojom::ImageFormat::JPEG:
-      if (gfx::JPEGCodec::Encode(bitmap, kDefaultQuality, &data)) {
+      if (gfx::JPEGCodec::Encode(bitmap, kDefaultQuality, &data))
         thumbnail_data.swap(data);
-        break;
-      }
+      break;
   }
   callback.Run(thumbnail_data, original_size);
 }