Remove remaining uses of Skia private header includes. (#5570)

Part of fixing https://bugs.chromium.org/p/skia/issues/detail?id=7741

shell/common/rasterizer.cc

@@ -11,7 +11,7 @@
 #include "third_party/skia/include/core/SkPictureRecorder.h"
 #include "third_party/skia/include/core/SkSurface.h"
 #include "third_party/skia/include/core/SkSurfaceCharacterization.h"
-#include "third_party/skia/src/utils/SkBase64.h"
+#include "third_party/skia/include/utils/SkBase64.h"
 
 namespace shell {
 

shell/common/skia_event_tracer_impl.cc

@@ -12,7 +12,7 @@
 #include "lib/fxl/logging.h"
 #include "third_party/dart/runtime/include/dart_tools_api.h"
 #include "third_party/skia/include/utils/SkEventTracer.h"
-#include "third_party/skia/src/core/SkTraceEventCommon.h"
+#include "third_party/skia/include/utils/SkTraceEventPhase.h"
 
 namespace skia {