The 'gn gen' step in roll_aosp.sh is failing due to trying to access
third_party/rapidjson/BUILD.gn, which isn't in AOSP. The rapidJSON
library is required to build the ANGLE libraries with frame capture
enabled, which isn't necessary for AOSP builds.

This CL adds the GN arg 'angle_has_rapidjson' which roll_aosp.sh will
set to 'false' to prevent 'gn gen' from attempting to access
third_party/rapidjson/BUILD.gn.

Bug: b/187342779
Change-Id: I6fcc0e94b6325be7523e35f966b8e4dbc708e3b0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2973338Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tim Van Patten <timvp@google.com>