JNI/Mojom generators: insert into sys.path instead of appending

This fixes a couple of build failures happening on a class of Python
installs.

I must say I can't quite pin down what characteristics in a Python
installation would influence this, but it certainly does on all
my machines that I setup to be non-corp machines to try and mimic
what open source developers may do.

Change-Id: Id60cd8b10141ca1486e0cdadfc7392b6d2bba3e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2445878
Commit-Queue: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813821}

base/android/jni_generator/jni_generator.py

@@ -27,7 +27,9 @@ _FILE_DIR = os.path.dirname(__file__)
 _CHROMIUM_SRC = os.path.join(_FILE_DIR, os.pardir, os.pardir, os.pardir)
 _BUILD_ANDROID_GYP = os.path.join(_CHROMIUM_SRC, 'build', 'android', 'gyp')
 
-sys.path.append(_BUILD_ANDROID_GYP)
+# Item 0 of sys.path is the directory of the main file; item 1 is PYTHONPATH
+# (if set); item 2 is system libraries.
+sys.path.insert(1, _BUILD_ANDROID_GYP)
 
 from util import build_utils
 

mojo/public/tools/bindings/generators/mojom_java_generator.py

@@ -20,9 +20,12 @@ import mojom.generate.generator as generator
 import mojom.generate.module as mojom
 from mojom.generate.template_expander import UseJinja
 
-sys.path.append(os.path.join(os.path.dirname(__file__), os.pardir,
-                             os.pardir, os.pardir, os.pardir, os.pardir,
-                             'build', 'android', 'gyp'))
+# Item 0 of sys.path is the directory of the main file; item 1 is PYTHONPATH
+# (if set); item 2 is system libraries.
+sys.path.insert(
+    1,
+    os.path.join(os.path.dirname(__file__), os.pardir, os.pardir, os.pardir,
+                 os.pardir, os.pardir, 'build', 'android', 'gyp'))
 from util import build_utils