Fix JNI generation dependency in android_webview_unittests.

In the current gyp rules, the unittests depend on the following three
children: (1) java, (2) native and (3) jni generation. This is not
correct as (2) has a functional dependency on (2). Having (3) being a
sibling of (2), as it is now, introduces a race when building the
unittest target from clean.
This change fixes the problem making the jni generation step a
dependency of the native build, rather than a sibling.

BUG=None
TEST=ninja -C out/Debug android_webview_unittests_apk
NOTRY=true

Review URL: https://codereview.chromium.org/116043010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243426 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243426