Revert "[Runtime] Disable extensions api for external web page."

It broke several Android device tests:

* org.xwalk.runtime.client.test.DeviceCapabilitiesTest#testDeviceCapabilities
* org.xwalk.runtime.client.test.ExternalExtensionTest#testExternalExtensionAsync
* org.xwalk.runtime.client.test.PresentationTest#testPresentationDisplayAvailable
* org.xwalk.runtime.client.test.ExternalExtensionTest#testExternalExtensionSync

* org.xwalk.runtime.client.embedded.test.PresentationTest#testPresentationDisplayAvailable
* org.xwalk.runtime.client.embedded.test.ExternalExtensionTest#testExternalExtensionAsync
* org.xwalk.runtime.client.embedded.test.DeviceCapabilitiesTest#testDeviceCapabilities
* org.xwalk.runtime.client.embedded.test.ExternalExtensionTest#testExternalExtensionSync

* org.xwalk.core.xwview.test.ExtensionEchoTest#testSync
* org.xwalk.core.xwview.test.LoadTest#testContentUrl
* org.xwalk.core.xwview.test.ExtensionEchoTest#testMultiFrames
* org.xwalk.core.xwview.test.ShouldInterceptLoadRequestTest#testNotCalledForExistingContentUrl
* org.xwalk.core.xwview.test.ExtensionEchoTest#testAsync

* org.xwalk.core.internal.xwview.test.ExtensionEchoInternalTest#testAsync
* org.xwalk.core.internal.xwview.test.ExtensionEchoInternalTest#testSync
* org.xwalk.core.internal.xwview.test.ExtensionBroadcastInternalTest#test
* org.xwalk.core.internal.xwview.test.ExtensionEchoInternalTest#testMultiFrames
* org.xwalk.core.internal.xwview.test.LoadTest#testContentUrl

This reverts commit 08afec43d74cfa2b586b7297bb7fe60543552bf6.