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### Platforms affected
Android
iOS (both UIWebView & WKWebView implementations)
### What does this PR do?
Adds support for [postMessage API](https://developer.mozilla.org/en-US/docs/Web/API/Window/postMessage) enabling pages loaded into the InappBrowser to post messages back to the parent Webview of the Cordova app.
For example, sending event messages associated with UI interactions such as button clicks from the wrapped page back to the parent app Webview.
### What testing has been done on this change?
Automated tests have been extended to cover the `message` event.
### Checklist
- [x ] Commit message follows the format: "GH-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
- [ x] Added automated test coverage as appropriate for this change.
For both UIWebView and WKWebView implementations on iOS.
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### Platforms affected
iOS
### What does this PR do?
Fixes `beforeload` event (introduced by #276) for iOS
### What testing has been done on this change?
Tested both allow & deny loading of URL with both iOS implementations in [test container app](https://github.com/dpa99c/cordova-plugin-inappbrowser-wkwebview-test) (ignore its README).
- To test with UIWebView use options: `beforeload=yes,usewkwebview=no`
- To test with WKWebView use options: `beforeload=yes,usewkwebview=yes`
### Checklist
- [x] [Reported an issue](http://cordova.apache.org/contribute/issues.html) in the JIRA database
- [x] Commit message follows the format: "CB-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
- [x] Added automated test coverage as appropriate for this change.
closes#349
### Platforms affected
iOS
### What does this PR do?
fix issue[ #340](https://github.com/apache/cordova-plugin-inappbrowser/issues/340) iOS CDVWKInAppBrowser evaluateJavascript method randomly gets blocked on iOS 12
### What testing has been done on this change?
manual testing
### Checklist
- [x] [Reported an issue](http://cordova.apache.org/contribute/issues.html) in the JIRA database
- [x] Commit message follows the format: "CB-3232: (android) Fix bug with resolving file paths", where CB-xxxx is the JIRA ID & "android" is the platform affected.
- [ ] Added automated test coverage as appropriate for this change.
Dave Alden