ReadableByteStreamControllerPullInto should always transfer the given buffer

[tschneidereit]

Currently, the buffer is not transferred if the bodies of steps 7 or 8 (sub-steps a or b) are hit.

For step 8, that's maybe not too bad, because the buffer's contents are copied and the buffer itself isn't exposed anyway. For step 7 we do expose the non-transferred buffer as the chunk value's underlying buffer.

In any case, it seems inconsistent for whether the buffer will be transferred or not to depend on the exact results of the pullInto operation.

Is there any reason not to just transfer the buffer before step 5?

cc @tyoshino @domenic