fix bug alert() iOS

[Luo-Kuang]

Platforms affected

iOS

Motivation and Context

issues #1120

Description

Testing

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• [x ] Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• [x ] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• [x ] I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

Merging #1121 (1eee12b) into master (8a6569c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1121   +/-   ##
=======================================
  Coverage   74.86%   74.86%           
=======================================
  Files          13       13           
  Lines        1723     1723           
=======================================
  Hits         1290     1290           
  Misses        433      433           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8a6569c...1eee12b. Read the comment docs.

[dpogue]

This looks okay to me. I'm not sure it needs to be a class method rather than an instance method, but the idea seems good

[dpogue]

Is there a reason for this to be a static method on CDVWebViewUIDelegate rather than an instance method used with [self topViewController]? It looks like all uses of it are within the CDVWebViewUIDelegate class