Make tools/run_hooks.py work when depot_tools is not in PATH

It should be possible to create and build from a Mojo checkout even
when depot_tools is not in the PATH, even though many common
development workflows require it. This teaches tools/run_hooks.py to
add a default depot_tools location to the PATH that hooks run in so
that a jiri checkout can execute hooks without PATH modifications.

R=kulakowski@chromium.org, lanechr@google.com, viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1760353002 .

tools/run_hooks.py

diff --git a/tools/run_hooks.py b/tools/run_hooks.py
index 478391b..52e27a0 100755
--- a/tools/run_hooks.py
+++ b/tools/run_hooks.py
@@ -23,11 +23,18 @@
   with open(deps_path) as deps_contents:
     d = deps_contents.read()
   exec(d, scope)
+  env = os.environ
+  # Some hooks expect depot_tools to be in the PATH already so add the default
+  # location of depot_tools in a standard checkout to the environment the hooks
+  # run in.
+  default_depot_tools_path = os.path.abspath(os.path.join(gclient_path,
+      "depot_tools"))
+  env["PATH"] += os.pathsep + default_depot_tools_path
   for hook in scope["hooks"]:
     name = hook["name"]
     print "________ running '%s' in '%s'" % (" ".join(hook["action"]),
         gclient_path)
-    subprocess.check_call(hook["action"], cwd=gclient_path)
+    subprocess.check_call(hook["action"], cwd=gclient_path, env=env)
     print
   return 0