Reland "Add ETC2 support to Metal backend."

This is a reland of c25802db30fa371d3984eeef6b04a605e7b7f51f

Original change's description:
> Add ETC2 support to Metal backend.
> 
> Fills out onCreateCompressedTexture and sets iOS caps to support ETC2.
> Skia supports no compressed texture formats on MacOS as yet.
> 
> Bug: skia:8243
> Change-Id: I2ce20f601c035a8822e658c88b815fdd8587aa98
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/240692
> Commit-Queue: Jim Van Verth <jvanverth@google.com>
> Reviewed-by: Brian Salomon <bsalomon@google.com>

Bug: skia:8243
Change-Id: Idebc46f29f4b75d216cf877606af290b0b3b8318
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/242080
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
diff --git a/src/gpu/gl/GrGLTexture.cpp b/src/gpu/gl/GrGLTexture.cpp
index 5473980..a5a1a37 100644
--- a/src/gpu/gl/GrGLTexture.cpp
+++ b/src/gpu/gl/GrGLTexture.cpp
@@ -50,7 +50,7 @@
         , fParameters(sk_make_sp<GrGLTextureParameters>()) {
     this->init(desc);
     this->registerWithCache(budgeted);
-    if (GrPixelConfigIsCompressed(desc.fConfig)) {
+    if (GrGLFormatIsCompressed(desc.fFormat)) {
         this->setReadOnly();
     }
 }