WebNN: Implement MLBuffer dispatch

Adds support to execute MLGraphs using MLBuffers. Allows the WebNN
developer to directly bind MLBuffers as input/outputs to graphs
for execution, which keeps MLBuffer data on-device after execution
completes. In future CLs, dispatch can be further optimized.

* Moves out per graph resources required by Dispatch().
* MLGraphBuilder.build no longer pre-allocates I/O.

webmachinelearning/webnn#482

Bug: 1472888
Change-Id: I7400704cf60c149c47c20f22c50d5f12bff89cf9
Cq-Include-Trybots: luci.chromium.try:win11-blink-rel

Author: bbernhar

webnn/conformance_tests/buffer.https.any.js

@@ -14,3 +14,5 @@ testDestroyWebNNBuffer('destroyTwice');
 testReadWebNNBuffer('read');
 
 testWriteWebNNBuffer('write');
+
+testDispatchWebNNBuffer('dispatch');

webnn/conformance_tests/gpu/buffer.https.any.js

@@ -14,3 +14,5 @@ testDestroyWebNNBuffer('destroyTwice', 'gpu');
 testReadWebNNBuffer('read', 'gpu');
 
 testWriteWebNNBuffer('write', 'gpu');
+
+testDispatchWebNNBuffer('dispatch', 'gpu');