Sometimes we wish to pull in a complicated dependency but want to
suppress or replace one or more of the hooks rules.  Say for example
we want to use a different way of generating the projects, or
have a different set of landmine expectations.

Here we add a custom_hooks section mirroring custom_deps to allow us to
override sections we have identified in the DEPS file.  To do so,
we add an optional name to the elements of the hooks list, and overwrite
those whose name matches.

Conventions between included DEPS and the .gclient as to the meanings of the
name are equivalent to the meaning of the customized deps, and so do not
benefit from further structure or definition.

BUG=None
TEST=local unit test

Review URL: https://chromiumcodereview.appspot.com/17742004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@210573 0039d316-1c4b-4281-b951-d872f2087c98