webgpu: Enable importExternalTexture (tensorflow#7976)

BUG
* webgpu: Enable importExternalTexture

Issues related to importExternalTexture were fixed in Chrome, so that we
may enable this feature by default again.
Note that its Linux implementation still has problem, and WebGPU support
on Linux hasn't been officially released yet.
Bug tensorflow#7972

* Fix the test case though it's not enabled

tfjs-backend-webgpu/src/from_pixels_webgpu_test.ts

@@ -50,7 +50,6 @@ describeWebGPU('fromPixels', () => {
         document.body.appendChild(video);
         await test_util.play(video);
 
-        // importExternalTexture is temporarily disabled
         {
           tf.env().set('WEBGPU_IMPORT_EXTERNAL_TEXTURE', true);
           const res = tf.browser.fromPixels(video);
@@ -59,7 +58,7 @@ describeWebGPU('fromPixels', () => {
           expect(data.length).toEqual(90 * 160 * 3);
           const freeTexturesAfterFromPixels =
               textureManager.getNumFreeTextures();
-          expect(freeTexturesAfterFromPixels).toEqual(1);
+          expect(freeTexturesAfterFromPixels).toEqual(0);
           const usedTexturesAfterFromPixels =
               textureManager.getNumUsedTextures();
           expect(usedTexturesAfterFromPixels).toEqual(0);

tfjs-backend-webgpu/src/kernels/FromPixels.ts

@@ -63,10 +63,8 @@ export function fromPixels(args: {
       [pixels.width, pixels.height];
   const outputShape = [height, width, numChannels];
 
-  // Disable importExternalTexture temporarily as it has problem in spec and
-  // browser impl
   const importVideo =
-      false && env().getBool('WEBGPU_IMPORT_EXTERNAL_TEXTURE') && isVideo;
+      env().getBool('WEBGPU_IMPORT_EXTERNAL_TEXTURE') && isVideo;
   const isVideoOrImage = isVideo || isImage;
   if (isImageBitmap || isCanvas || isVideoOrImage) {
     let resource;