Layout tests: Enable fetching generated files from the build directory

content/shell/common/layout_test.mojom

Index: content/shell/common/layout_test.mojom
diff --git a/content/shell/common/layout_test.mojom b/content/shell/common/layout_test.mojom
index c0de4b5e61ab2b3273bfad4e5fc5b08f90243c96..81f97031250652eeb175260b0acea97883040ada 100644
--- a/content/shell/common/layout_test.mojom
+++ b/content/shell/common/layout_test.mojom
@@ -15,6 +15,9 @@ struct ShellTestConfiguration {
   // The temporary directory of the system.
   mojo.common.mojom.FilePath temp_path;
 
+  // The build directory.
+  mojo.common.mojom.FilePath build_directory;
+

[qyearsley, 2017/03/20 17:40:00]

In general, what does this file control? Is ShellTestConfiguration something
that is always set up when running content_shell?

[yzshen1, 2017/03/20 18:01:04]

This is the layout test configuration information sending from the browser to
the renderer process.

It is only set up when running layout tests.

   // The URL of the current layout test.
   url.mojom.Url test_url;