media/gpu: Make VaapiImageDecoder explicit ctor

Micro nit CL.

Bug: 877694
Test: jpeg_decode_accelerator_unittest
Change-Id: Id7d8321615e74d36668fc8b1ef2bd54703da6967
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1669914
Commit-Queue: Gil Dekel <gildekel@chromium.org>
Commit-Queue: Andres Calderon Jaramillo <andrescj@chromium.org>
Auto-Submit: Gil Dekel <gildekel@chromium.org>
Reviewed-by: Andres Calderon Jaramillo <andrescj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671358}

media/gpu/vaapi/vaapi_image_decoder.h

@@ -66,7 +66,7 @@ class VaapiImageDecoder {
       VaapiImageDecodeStatus* status) = 0;
 
  protected:
-  VaapiImageDecoder(VAProfile va_profile);
+  explicit VaapiImageDecoder(VAProfile va_profile);
 
   scoped_refptr<VaapiWrapper> vaapi_wrapper_;