Link to web_platform_tests.md where appropriate

docs/testing/writing_layout_tests.md

 # Writing Layout Tests
 
 _Layout tests_ is a bit of a misnomer. This term is
 [a part of our WebKit heritage](https://webkit.org/blog/1452/layout-tests-theory/),
 and we use it to refer to every test that is written as a Web page (HTML, SVG,
 or XHTML) and lives in
 [third_party/WebKit/LayoutTests/](../../third_party/WebKit/LayoutTests).
 
 [TOC]
 
 ## Overview
 
 Layout tests should be used to accomplish one of the following goals:
 
 1. The entire surface of Blink that is exposed to the Web should be covered by
    tests that we contribute to the
-   [Web Platform Tests Project](https://github.com/w3c/web-platform-tests)
-   (WPT). This helps us avoid regressions, and helps us identify Web Platform
-   areas where the major browsers don't have interoperable implementations.
-   Furthermore, by contributing to projects such as WPT, we share the burden of
-   writing tests with the other browser vendors, and we help all the browsers
-   get better. This is very much in line with our goal to move the Web forward.
+   [Web Platform Tests Project](./web_platform_tests.md) (WPT). This helps us
+   avoid regressions, and helps us identify Web Platform areas where the major
+   browsers don't have interoperable implementations. Furthermore, by
+   contributing to projects such as WPT, we share the burden of writing tests
+   with the other browser vendors, and we help all the browsers get better. This
+   is very much in line with our goal to move the Web forward.
 2. When a Blink feature cannot be tested using the tools provided by WPT, and
    cannot be easily covered by
    [C++ unit tests](https://cs.chromium.org/chromium/src/third_party/WebKit/Source/web/tests/?q=webframetest&sq=package:chromium&type=cs),
    the feature must be covered by layout tests, to avoid unexpected regressions.
    These tests will use Blink-specific testing APIs that are only available in
    [content_shell](./layout_tests_in_content_shell.md).
 
 *** promo
 If you know that Blink layout tests are upstreamed to other projects, such as
 [test262](https://github.com/tc39/test262), please update this document. Most
@@ skipping 546 matching lines @@
 * The `http/` directory hosts tests that require an HTTP server (see above).
 * The `resources/` subdirectory in every directory contains binary files, such
   as media files, and code that is shared by multiple test files.
 
 *** note
 Some layout tests consist of a minimal HTML page that references a JavaScript
 file in `resources/`. Please do not use this pattern for new tests, as it goes
 against the minimality principle. JavaScript and CSS files should only live in
 `resources/` if they are shared by at least two test files.
 ***