Turn off mip map support in GrDawnCaps

GrDawnGpu::onRegenerateMipMapLevels() is not implemented. Set mip map
support to false in GrDawnCaps so that the function is not called.

Change-Id: I342c59d171d21250ee4da400f58b6da54a04ca8c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/272139
Commit-Queue: Sean Gilhuly <sgilhuly@chromium.org>
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Stephen White <senorblanco@chromium.org>

Author: Sean Gilhuly

src/gpu/dawn/GrDawnCaps.cpp

@@ -14,7 +14,7 @@
 #include "src/gpu/GrStencilSettings.h"
 
 GrDawnCaps::GrDawnCaps(const GrContextOptions& contextOptions) : INHERITED(contextOptions) {
-    fMipMapSupport = true;
+    fMipMapSupport = false;  // FIXME: implement onRegenerateMipMapLevels in GrDawnGpu.
     fBufferMapThreshold = SK_MaxS32;  // FIXME: get this from Dawn?
     fShaderCaps.reset(new GrShaderCaps(contextOptions));
     fMaxTextureSize = fMaxRenderTargetSize = 4096; // FIXME