Fix monochrome_public_apk_checker attempt #4

This CL:
- Adds dependency from monochrome_public_apk_checker to
build/android:devil_chromium_py gn target
- Makes monochrome_apk_checker_test.py ignore the pathmap file if it does
not exist. In local testing no path map is generated for
SystemWebView.apk

BUG=1138758

Change-Id: Ie087fe20117c95fe0acadf35bb9dc06d1614164b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2485280Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818921}

chrome/android/monochrome/BUILD.gn

@@ -10,6 +10,7 @@ python_library("monochrome_public_apk_checker") {
       "//chrome/android/monochrome/scripts/monochrome_python_tests.pydeps"
   data_deps = [
     "//android_webview:system_webview_apk",
+    "//build/android:devil_chromium_py",
     "//chrome/android:chrome_public_apk",
     "//chrome/android:monochrome_public_apk",
     "//testing:run_isolated_script_test",

chrome/android/monochrome/scripts/monochrome_apk_checker_test.py

@@ -229,7 +229,7 @@ However these files were present in {0} but not in Monochrome:
     Returns: A dict mapping from obfuscated paths to original paths or an
            empty dict if passed a None |pathmap_path|.
     """
-    if pathmap_path is None:
+    if pathmap_path is None or not os.path.exists(pathmap_path):
       return {}
 
     pathmap = {}