Change call-sites now that SkCanvas is not ref-counted

ui/surface/transport_dib_win.cc

Index: ui/surface/transport_dib_win.cc
diff --git a/ui/surface/transport_dib_win.cc b/ui/surface/transport_dib_win.cc
index 190065b9ded3acff714154ce7456a7362b218fb6..baa96cbc2a7d3e235b12236d597fcf10c4e088d0 100644
--- a/ui/surface/transport_dib_win.cc
+++ b/ui/surface/transport_dib_win.cc
@@ -58,8 +58,9 @@ bool TransportDIB::is_valid_handle(Handle dib) {
   return dib.IsValid();
 }
 
-SkCanvas* TransportDIB::GetPlatformCanvas(int w, int h,
-                                                      bool opaque) {
+std::unique_ptr<SkCanvas> TransportDIB::GetPlatformCanvas(int w,
+                                                          int h,
+                                                          bool opaque) {
   // This DIB already mapped the file into this process, but PlatformCanvas
   // will map it again.
   DCHECK(!memory()) << "Mapped file twice in the same process.";