Updated depot_tools link (flutter#4498)

CONTRIBUTING.md

@@ -23,7 +23,7 @@ Things you will need
  * git (used for source version control).
  * An IDE. We recommend [IntelliJ with the Flutter plugin](https://flutter.io/intellij-ide/) or Xcode.
  * An ssh client (used to authenticate with GitHub).
- * Chromium's [depot_tools](http://www.chromium.org/developers/how-tos/install-depot-tools) (make sure it's in your path). We use the `gclient` tool from depot_tools.
+ * Chromium's [depot_tools](http://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html#_setting_up) (make sure it's in your path). We use the `gclient` tool from depot_tools.
  * Python (used by many of our tools, including 'gclient').
  * On Mac OS X and Linux: curl and unzip (used by `gclient sync`).
  * On Windows: Visual Studio (required for non-Googlers only).