fix for GrAtlasTextContext occasionally crashes on mixed runs

TBR=bsalomon@google.com
BUG=510931

Review URL: https://codereview.chromium.org/1252423002
diff --git a/src/gpu/GrAtlasTextContext.cpp b/src/gpu/GrAtlasTextContext.cpp
index 4c1ef04..8725aa5 100644
--- a/src/gpu/GrAtlasTextContext.cpp
+++ b/src/gpu/GrAtlasTextContext.cpp
@@ -1265,7 +1265,7 @@
 
     PerSubRunInfo* subRun = &run.fSubRunInfo.back();
     if (run.fInitialized && subRun->fMaskFormat != format) {
-        subRun = &run.fSubRunInfo.push_back();
+        subRun = &run.push_back();
     }
 
     run.fInitialized = true;