Link to new web-platform-tests.org documentation from our own

Many pages of testthewebforward.org now redirect, so replace links with
something appropriate from the new documentation.

Since the new documentation consistently says "web-platform-tests"
instead of "Web Platform Tests" and this is what folks like jgraham tend
to write as well, align our own usage with that.

BUG=657117

Review-Url: https://codereview.chromium.org/2696323003
Cr-Commit-Position: refs/heads/master@{#451045}
diff --git a/docs/testing/layout_tests_tips.md b/docs/testing/layout_tests_tips.md
index 8a2ee128..cd50bcc 100644
--- a/docs/testing/layout_tests_tips.md
+++ b/docs/testing/layout_tests_tips.md
@@ -20,7 +20,7 @@
 ## General Principles
 
 This section contains guidelines adopted from
-[Test the Web Forward's Test Format Guidelines](http://testthewebforward.org/docs/test-format-guidelines.html)
+[web-platform-tests documentation](http://web-platform-tests.org/writing-tests/general-guidelines.html)
 and
 [WebKit's Wiki page on Writing good test cases](https://trac.webkit.org/wiki/Writing%20Layout%20Tests%20for%20DumpRenderTree),
 with Blink-specific flavoring.
@@ -93,7 +93,7 @@
 `testharness.js` makes a test self-describing when used correctly. Other types
 of tests, such as reference tests and
 [tests with manual fallback](./layout_tests_with_manual_fallback.md),
-[must be carefully designed](http://testthewebforward.org/docs/test-style-guidelines.html)
+[must be carefully designed](http://web-platform-tests.org/writing-tests/manual.html#requirements-for-a-manual-test)
 to be self-describing.
 
 ### Minimal