media/gpu/vaapi: add a DCHECK and stringify a log

Just adding a DCHECK and using a stringify function in a log.
No functionality change intended.

Bug: b:162962069
Change-Id: Ie7f7520fe769405342bc21a15c810bb9c35eef8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367444
Auto-Submit: Miguel Casas <mcasas@chromium.org>
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800318}

media/gpu/vaapi/vaapi_wrapper.cc

@@ -745,8 +745,8 @@ bool AreAttribsSupported(const base::Lock* va_lock,
     if (attribs[i].type != required_attribs[i].type ||
         (attribs[i].value & required_attribs[i].value) !=
             required_attribs[i].value) {
-      DVLOG(1) << "Unsupported value " << required_attribs[i].value
-               << " for attribute type " << required_attribs[i].type;
+      DVLOG(1) << "Unsupported value " << required_attribs[i].value << " for "
+               << vaConfigAttribTypeStr(required_attribs[i].type);
       return false;
     }
   }
@@ -2151,6 +2151,7 @@ bool VaapiWrapper::BlitSurface(const VASurface& va_surface_src,
                                base::Optional<gfx::Rect> src_rect,
                                base::Optional<gfx::Rect> dest_rect,
                                VideoRotation rotation) {
+  DCHECK_EQ(mode_, kVideoProcess);
   base::AutoLock auto_lock(*va_lock_);
 
   // Create a buffer for VPP if it has not been created.