Add doc instructions for multi-repo Chromium trybots.

site/dev/chrome/multi_repo_trybots.md

+Multiple repo Chromium trybots
+==============================
+
+When a proposed Skia change will require a change in Chromium or Blink it is
+often helpful to locally create the Chromium and Blink changes and test with the
+proposed Skia change. This often happens with Skia API changes and changes
+which affect Blink layout tests. While simple to do locally, this explains how
+to do so on the Chromium trybots.
+
+The basic idea is to make your Chromium and Blink change in the usual way, but
+then pull in other changes by modifying the \<chromium>/src/DEPS file.
+
+
+Rietveld
+--------
+If the the patch to be applied is to a project already in Chromium (like Skia)
+and the patch is already in Rietveld, then add the following to
+\<chromium>/src/DEPS in the 'hooks' array just before the 'gyp' hook.
+
+      {
+        'name': 'apply_custom_patch',
+        'pattern': '.',
+        'action': ['apply_issue',
+                   '--root_dir', 'src/third_party/skia',
+                   '--issue', '1873923002',
+                   '--patchset', '160001',

[dogben, 2016/04/14 16:40:51]

Maybe indicate that issue, patchset, and potentially server may need to be
replaced? Also, for someone not familiar with Reitveld, it's not at all clear
how to find the correct value for patchset.

[bungeman-skia, 2016/04/14 17:16:12]

Eck, I actually wrote that but it appears I didn't save it. I'll add it.
Unfortunately, Rietveld is quite bad at exposing the actual patchset number.

+                   '--server', 'https://codereview.chromium.org',
+                   '--force',
+                   '--ignore_deps',
+                   '-v',
+                   '-v',
+                   '--no-auth',
+                   '--blacklist', 'DEPS'
+        ],
+      },
+
+An example of this being used can be seen at
+https://crrev.com/1877673002/#ps120001 .
+
+
+External changes not in rietveld
+--------------------------------
+If the patch is to files where the above is not possible, then it is still
+possible to patch the files manually by adding the following to
+\<chromium>/src/DEPS in the 'hooks' array just before the 'gyp' hook.
+
+      {
+        'name': 'apply_custom_patch',
+        'pattern': '.',
+        'action': ['python',
+                   '-c', 'from distutils.dir_util import copy_tree; copy_tree("src/patch/", "src/");'
+        ],
+      },
+
+Then, copy all 'out of tree' files into \<chromium>/src/patch/, using the same
+directory structure used by Chromium. When 'gclient runhooks' is run, the files
+in \<chromium>/src/patch/ will be copied to and overwrite corresponding files in
+\<chromium>/src/. For example, if changing \<skia>/include/core/SkPath.h, place
+a copy of the modified SkPath.h at
+\<chromium>/src/patch/third_party/skia/include/core/SkPath.h.
+
+An example of this being used can be seen at
+https://crrev.com/1866773002/#ps20001 .
+
+
+Try the patch
+-------------
+After committing these \<chromium>/src/DEPS and \<chromium>/src/patch/ changes
+locally, 'git cl upload' can be used in the usual way. Be sure to add
+'COMMIT=false' to the issue description to avoid accidentally checking it in.