Use a pseudo-extension CHROMIUM_framebuffer_mixed_samples

src/gpu/gl/GrGLInterface.cpp

Index: src/gpu/gl/GrGLInterface.cpp
diff --git a/src/gpu/gl/GrGLInterface.cpp b/src/gpu/gl/GrGLInterface.cpp
index 9cdc9f438657d3bee1c2c3cea9738805bbdb5723..b8d7db6b2e513bcfe3c16b778cef36601954893a 100644
--- a/src/gpu/gl/GrGLInterface.cpp
+++ b/src/gpu/gl/GrGLInterface.cpp
@@ -541,7 +541,8 @@ bool GrGLInterface::validate() const {
         }
     }
 
-    if (fExtensions.has("GL_NV_framebuffer_mixed_samples")) {
+    if (fExtensions.has("GL_NV_framebuffer_mixed_samples") ||
+        fExtensions.has("GL_CHROMIUM_framebuffer_mixed_samples")) {
         if (nullptr == fFunctions.fCoverageModulation) {
             RETURN_FALSE_INTERFACE
         }