Use the correct value for Process.WEBVIEW_ZYGOTE_UID.

When the libcutils constant was added there a merge conflict, which
caused the AID_WEBVIEW_ZYGOTE value to land with a different value than
the Java-side Process value. Nothing yet uses the Process constant, so
there were no ill effects.

Test: m
Change-Id: I8cc87bce1ddbdcdaf79d85c828d86837e96cce21