[Example] fail to run PoseNet when only selecting WebML backend

[Christywl]

Test Env: Chromium Version: nightly build 75.0.3739.0 (c623981) Platform: macOS 10.13.6

Expected Result: The example should work when only choosing WebML backend.

Actual Result: Fail to run PoseNet when only selecting WebML backend, the URL is: http://localhost:8080/examples/skeleton_detection/?prefer=sustained&b=none&s=image&d=0 (b is none when only selecting WebML backend):

If selecting WASM + SUSTAINED, and then un-selected WASM, SUSTAINED works, the URL is: http://localhost:8080/examples/skeleton_detection/?prefer=sustained&b=WebML&s=image&d=0 (b is WebML when only selecting WebML backend)

How to Reproduce:

1. Setup test server and start
2. Visit http://localhost:8080/examples/skeleton_detection, select SUSTAINED_SPEED --> doesn't work
3. Visit http://localhost:8080/examples/skeleton_detection, select WASM + SUSTAINED_SPEED, and then un-selected WASM --> work

[Christywl]

This issue was fixed on the webml-polyfill commit https://github.com/intel/webml-polyfill/commit/27851f9809e816385200bb87f07d16a40100551f.