Update moved find_depot_tools.py script location

The script moved from /tools to /build in
https://codereview.chromium.org/1418513007
and we haven't noticed due to cached .pyc files.

We could add a cleaning of .pyc files similar to
https://codereview.chromium.org/1186593003 but I think
that's overkill for WebRTC.

BUG=
R=phoglund@webrtc.org

Review URL: https://codereview.webrtc.org/1409353014 .

Cr-Commit-Position: refs/heads/master@{#10526}

.gitignore

@@ -137,7 +137,6 @@
 /third_party/zlib
 /tools/android
 /tools/clang
-/tools/find_depot_tools.py
 /tools/generate_library_loader
 /tools/gn
 /tools/grit

setup_links.py

@@ -102,7 +102,6 @@ if 'ios' in target_os:
 
 FILES = {
   'chrome/VERSION': None,  # TODO(kjellander): Get rid of this; webrtc:5160.
-  'tools/find_depot_tools.py': None,
   'tools/isolate_driver.py': None,
   'third_party/BUILD.gn': None,
 }

tools/autoroller/roll_chromium_revision.py

@@ -38,7 +38,7 @@ CHECKOUT_ROOT_DIR = os.path.realpath(os.path.join(SCRIPT_DIR, os.pardir,
 sys.path.append(CHECKOUT_ROOT_DIR)
 import setup_links
 
-sys.path.append(os.path.join(CHECKOUT_ROOT_DIR, 'tools'))
+sys.path.append(os.path.join(CHECKOUT_ROOT_DIR, 'build'))
 import find_depot_tools
 find_depot_tools.add_depot_tools_to_path()
 from gclient import GClientKeywords