Start using 20.0.0 of Android build tools.

BUG=393019

Review URL: https://codereview.chromium.org/387403002

git-svn-id: http://src.chromium.org/svn/trunk/src/build@283004 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -1618,7 +1618,7 @@
             'android_host_arch%': '<!(uname -m)',
             # Android API-level of the SDK used for compilation.
             'android_sdk_version%': '19',
-            'android_sdk_build_tools_version%': '19.0.0',
+            'android_sdk_build_tools_version%': '20.0.0',
             'host_os%': "<!(uname -s | sed -e 's/Linux/linux/;s/Darwin/mac/')",
           },
           # Copy conditionally-set variables out one scope.