Enable SUPPORTS_TEXT_BASED_API in WebKitLegacy for iOS

Enable SUPPORTS_TEXT_BASED_API in WebKitLegacy for iOS
https://bugs.webkit.org/show_bug.cgi?id=168919

Reviewed by Tim Horton.

Source/WebKit:

* WebKit.xcodeproj/project.pbxproj: Run scripts which are run for installhdrs phase for installapi phase as well.

Source/WebKit/mac:

* Configurations/WebKitLegacy.xcconfig: Enable SUPPORTS_TEXT_BASED_API.
* migrate-headers.sh: Run the migrate-headers script for installapi phase.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@213096 268f45cc-cd09-0410-ab3c-d52691b4dbfc