test_shell: fix for newer cairos

skia/ext/bitmap_platform_device_linux.cc

Index: skia/ext/bitmap_platform_device_linux.cc
diff --git a/skia/ext/bitmap_platform_device_linux.cc b/skia/ext/bitmap_platform_device_linux.cc
index f170273fc38ff0479fbabea2363918946732a02d..9b589de3254ffefd8cd1aa3d97d21fd2aa8a67d9 100644
--- a/skia/ext/bitmap_platform_device_linux.cc
+++ b/skia/ext/bitmap_platform_device_linux.cc
@@ -187,7 +187,11 @@ BitmapPlatformDevice::~BitmapPlatformDevice() {
 }
 
 cairo_t* BitmapPlatformDevice::beginPlatformPaint() {
-  return data_->GetContext();
+  cairo_t* cairo = data_->GetContext();
+  // Tell Cairo that we've (probably) modified its pixel buffer without
+  // its knowledge.
+  cairo_surface_mark_dirty(cairo_get_target(cairo));
+  return cairo;
 }
 
 void BitmapPlatformDevice::setMatrixClip(const SkMatrix& transform,