Add doc instructions for multi-repo Chromium trybots.

NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=1865153003

Review URL: https://codereview.chromium.org/1865153003

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+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',
+                   '--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.