Update links to layout test documentation.

docs/how_to_extend_layout_test_framework.md

 # How to Extend the Layout Test Framework
 
 The Layout Test Framework that Blink uses is a regression testing tool that is
 multi-platform and it has a large amount of tools that help test varying types
 of regression, such as pixel diffs, text diffs, etc. The framework is mainly
 used by Blink, however it was made to be extensible so that other projects can
 use it test different parts of chrome (such as Print Preview). This is a guide
 to help people who want to actually the framework to test whatever they want.
 
 [TOC]
 
 ## Background
 
 Before you can start actually extending the framework, you should be familiar
 with how to use it. This wiki is basically all you need to learn how to use it

[pwnall, 2016/11/15 18:23:11]

I wouldn't call it "wiki" anymore :)

[qyearsley, 2016/11/15 19:04:57]

Done, updated

-http://www.chromium.org/developers/testing/webkit-layout-tests
+https://chromium.googlesource.com/chromium/src/+/master/docs/testing/layout_tests.md
 
 ## How to Extend the Framework
 
 There are two parts to actually extending framework to test a piece of software.
 The first part is extending certain files in:
 [/third_party/Webkit/Tools/Scripts/webkitpy/layout_tests/](/third_party/Webkit/Tools/Scripts/webkitpy/layout_tests/)
 The code in `webkitpy/layout_tests` is the layout test framework itself
 
 The second part is creating a driver (program) to actually communicate the
 layout test framework. This part is significantly more tricky and dependant on
@@ skipping 225 matching lines @@
             *   In this case this is how your output should look.
                 *   “Content-type: image/png\n”
                 *   “ActualHash: hashData\n”
                 *   “Content-Length: lengthOfPng\n”
                 *   “pngdata”
                     *   This part doesn’t need a header specifying that you are
                         sending png data, just send it
                 *   “#EOF\n” on both stdout and stderr
             *   To see the structure of the data required, look at the
                 `read_block` functions in Driver.py