ServiceWorker: Add layout test for worker script MIME type.

LayoutTests/http/tests/serviceworker/resources/plain-text-worker.php

Index: LayoutTests/http/tests/serviceworker/resources/plain-text-worker.php
diff --git a/LayoutTests/http/tests/inspector/network/resources/script-as-text.php b/LayoutTests/http/tests/serviceworker/resources/plain-text-worker.php
similarity index 56%
copy from LayoutTests/http/tests/inspector/network/resources/script-as-text.php
copy to LayoutTests/http/tests/serviceworker/resources/plain-text-worker.php
index 2d318ecc3135616b2e0f22b92bf95b9f7722bb5e..696637597ef8463ca664b1ee6dc180d3a68add57 100644
--- a/LayoutTests/http/tests/inspector/network/resources/script-as-text.php
+++ b/LayoutTests/http/tests/serviceworker/resources/plain-text-worker.php
@@ -1,8 +1,3 @@
 <?php
     header("Content-Type: text/plain");
 ?>
-
-function foo()
-{
-    var bar = 42;
-}