[webml/NNAPI] Crash happened when attempting to do extra actions for the finished model

[Christywl]

Test Env: Chromium Version : nightly build 65.0.3324.0 (revision f9e1a31) OS : Android 8.1.0;Pixel 2

Expected Result: No crash happens

Actual Result: Crash happened when attempting to do extra actions for the finished model

How to reproduce:

Run test case on Chromium: https://brucedai.github.io/nt/testa/index-local.html

Test Cases:

1. raise error when attempting to add an operand into the finished model: https://github.com/intel/webml-polyfill/blob/master/test/unit-test/test-Model.js#L204
2. raise error when attempting to reset the value for an operand of the finished model: https://github.com/intel/webml-polyfill/blob/master/test/unit-test/test-Model.js#L903
3. raise error when attempting to reset the operation of the finished model:https://github.com/intel/webml-polyfill/blob/master/test/unit-test/test-Model.js#L3246
4. raise error when attempting to modify inputs/outputs of the finished model: https://github.com/intel/webml-polyfill/blob/master/test/unit-test/test-Model.js#L3347

[sunlin-link]

@BruceDai Please help to verify the fix.

[Christywl]

Verified it with Chromium nightly build(007f215), this issue can't reproduce, so close it.

[Christywl]

Reopen this issue since this issue occurs again on the Chromium nightly build 279911e 1fd54ed. Test URL: https://brucedai.github.io/nt/test/model.html?backend=nnapi

[sunlin-link]

regression by https://github.com/otcshare/chromium-src/pull/20

[sunlin-link]

@Christywl Fixed. Please verify

[Christywl]

Verified it with the chromium nightly build 3677b5f, this issue wasn't reproduced.