[Android] Add gn support for multidex.

build/android/gyp/apkbuilder.py

Index: build/android/gyp/apkbuilder.py
diff --git a/build/android/gyp/apkbuilder.py b/build/android/gyp/apkbuilder.py
index c44173f0f941f21c179ebd68e4bdd4938206a17b..317c3c439a29799b121f6397f0b4f54b001c5b52 100755
--- a/build/android/gyp/apkbuilder.py
+++ b/build/android/gyp/apkbuilder.py
@@ -146,7 +146,12 @@ def main(args):
           # ignored by md5_check.
           apk.writestr('lib/%s/libplaceholder.so' % options.android_abi, ':-)')
         if options.dex_file:
-          apk.write(options.dex_file, 'classes.dex')
+          if options.dex_file.endswith('.zip'):
+            with zipfile.ZipFile(options.dex_file, 'r') as dex_zip:
+              for dex in (d for d in dex_zip.namelist() if d.endswith('.dex')):
+                apk.writestr(dex, dex_zip.read(dex))
+          else:
+            apk.write(options.dex_file, 'classes.dex')
 
       shutil.move(tmp_apk, options.output_apk)
     finally: