Only set the ADB path if it exists.

Fixes domokit/devtools#60

BUG=
R=jeffbrown@google.com

Review URL: https://codereview.chromium.org/1781063002 .

Cr-Mirrored-From: https://github.com/domokit/mojo
Cr-Mirrored-Commit: 2b364b40eed465e50c58afc5901f36421ee3b698

devtoolslib/paths.py

diff --git a/devtoolslib/paths.py b/devtoolslib/paths.py
index 8c592ae..870dd0a 100644
--- a/devtoolslib/paths.py
+++ b/devtoolslib/paths.py
@@ -59,8 +59,10 @@
   params['build_dir_path'] = build_dir_path
   if is_android:
     params['shell_path'] = os.path.join(build_dir_path, 'apks', 'MojoShell.apk')
-    params['adb_path'] = os.path.join(root_path, 'third_party', 'android_tools',
-                                'sdk', 'platform-tools', 'adb')
+    adb_path = os.path.join(root_path, 'third_party', 'android_tools',
+                            'sdk', 'platform-tools', 'adb')
+    if os.path.isfile(adb_path):
+      params['adb_path'] = adb_path
   else:
     params['shell_path'] = os.path.join(build_dir_path, 'mojo_shell')