As soon as glslang is initialized (which is done when EGL is
initialized), try to compile a simple shader.  This will warm up
glslang's internals to avoid hitches on first shader compilation when a
WebGL application starts.

Potential future optimization is to use a worker thread to do this.

Bug: chromium:1091440
Change-Id: I6ac22e982d748af52a6e8b61239889425edc2edb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2280401Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>