Add option to specify ADB binary in test runner.

build/android/pylib/constants.py

Index: build/android/pylib/constants.py
diff --git a/build/android/pylib/constants.py b/build/android/pylib/constants.py
index 29da601259ddb734145583b41ca4e390fc75990e..da28eeb5d139ea9a6767e924ded00a8f887aae48 100644
--- a/build/android/pylib/constants.py
+++ b/build/android/pylib/constants.py
@@ -246,8 +246,21 @@ def _Memoize(func):
   return Wrapper
 
 
-@_Memoize
+def SetAdbPath(adb_path):
+  os.environ['ADB_PATH'] = adb_path
+
+
 def GetAdbPath():
+  # Check if a custom adb path as been set. If not, try to find adb
+  # on the system.
+  if os.environ.get('ADB_PATH'):
+    return os.environ.get('ADB_PATH')
+  else:
+    return _FindAdbPath()
+
+
+@_Memoize
+def _FindAdbPath():
   if os.environ.get('ANDROID_SDK_ROOT'):
     return 'adb'
   # If envsetup.sh hasn't been sourced and there's no adb in the path,
@@ -260,7 +273,6 @@ def GetAdbPath():
     logging.debug('No adb found in $PATH, fallback to checked in binary.')
     return os.path.join(ANDROID_SDK_ROOT, 'platform-tools', 'adb')
 
-
 # Exit codes
 ERROR_EXIT_CODE = 1
 WARNING_EXIT_CODE = 88