Default shaders will be compiled from source at runtime, this is because
they depend on ANGLE format table and there is currently no way to
pre-compile metal shaders in a cross-platform manner.

Using default shaders' source instead of pre-compiled form seems to
reduce the libGLESv2's binary size. However, the startup time will be
increased due to runtime cost of compilation, thus the compilation now
will be done asynchronously.

Bug: angleproject:5186
Change-Id: I0e1987d6c76692d5169255736fbb8e215185c33b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2482405Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>