This is helpful for implementing program binary caching in ANGLE.

in order to maintain the current behaviour in Chrome, we will need
to defer shader compilation until link time. Since WebGL allows the
app layer to enable and disable extensions explicitly, the app can
enable or disable an extension between the calls to glCompileShader
and glLinkProgram. Then, if we're deferring shader compilation, we
will have to preserve the extension settings at the time of the call
to compile, not the current settings at the time of link. Making the
compiler reference-counted gives us a pretty simple and robust
solution.

This also changes the Context class to lazily init the shader
compiler. Lazy init prevents any unexpected nulls, and also ensures
the releaseShaderCompiler does exactly that: just releases the shader
compiler, and does not immediately re-initialize it.

BUG=angleproject:2044

Change-Id: I0f0eeb4eb94dc4eee238f92482804a0921c77df0
Reviewed-on: https://chromium-review.googlesource.com/522868Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>