ServiceWorker: Tweak the repetitive registrations test

The "register-50-times" test case added in
https://codereview.chromium.org/851233003/ made the test slow (timeout
sometimes). This patch moves the test case to its own file, and reduces
the number of registration to 30. Also marks the new test as Slow.

BUG=448670

Review URL: https://codereview.chromium.org/872313002

git-svn-id: svn://svn.chromium.org/blink/trunk@188942 bbb929c8-8fbe-4397-9dbb-9b2b20218538
diff --git a/third_party/WebKit/LayoutTests/SlowTests b/third_party/WebKit/LayoutTests/SlowTests
index a52a8fd..89fd8c0 100644
--- a/third_party/WebKit/LayoutTests/SlowTests
+++ b/third_party/WebKit/LayoutTests/SlowTests
@@ -178,3 +178,4 @@
 crbug.com/372424 fast/dom/DOMImplementation/createDocument-with-used-doctype.html [ Slow ]
 
 crbug.com/372424 http/tests/serviceworker/chromium/registration-stress.html [ Slow ]
+crbug.com/448670 http/tests/serviceworker/chromium/register-different-script-many-times.html [ Slow ]