make GrTextBlob::subRunList() const

Change-Id: I02ffb7c09b1b1507700d618451a64bf89dd6953f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/374357
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
diff --git a/src/gpu/GrSurfaceDrawContext.cpp b/src/gpu/GrSurfaceDrawContext.cpp
index a9c1d81..dff2642 100644
--- a/src/gpu/GrSurfaceDrawContext.cpp
+++ b/src/gpu/GrSurfaceDrawContext.cpp
@@ -443,7 +443,7 @@
         }
     }
 
-    for (GrSubRun& subRun : blob->subRunList()) {
+    for (const GrSubRun& subRun : blob->subRunList()) {
         subRun.draw(clip, viewMatrix, glyphRunList, this);
     }
 }