Make GrAtlas::removePlot static

This CL just calves off a small chunk of (Add atlased layer purging - https://codereview.chromium.org/367073002/).

R=jvanverth@google.com
TBR=jvanverth@google.com

Author: robertphillips@google.com

Review URL: https://codereview.chromium.org/388103002
diff --git a/src/gpu/GrTextStrike.cpp b/src/gpu/GrTextStrike.cpp
index b290d6c..4079893 100644
--- a/src/gpu/GrTextStrike.cpp
+++ b/src/gpu/GrTextStrike.cpp
@@ -278,7 +278,7 @@
         }
     }
 
-    fAtlas->removePlot(&fPlotUsage, plot);
+    GrAtlas::RemovePlot(&fPlotUsage, plot);
 }