Conditionalize DeviceCGL implementation. Needed in order to build in WebKit. Bug: angleproject:3973 Tbr: geofflang@chromium.org Tbr: jmadill@chromium.org Change-Id: I9818a2f74d07daa4709892c1cfa81670e5f80617 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1851245 Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org>