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/GrAtlas.cpp b/src/gpu/GrAtlas.cpp
index 1bff42a..a9b8ae9 100644
--- a/src/gpu/GrAtlas.cpp
+++ b/src/gpu/GrAtlas.cpp
@@ -255,7 +255,7 @@
     return NULL;
 }
 
-void GrAtlas::removePlot(ClientPlotUsage* usage, const GrPlot* plot) {
+void GrAtlas::RemovePlot(ClientPlotUsage* usage, const GrPlot* plot) {
     int index = usage->fPlots.find(const_cast<GrPlot*>(plot));
     if (index >= 0) {
         usage->fPlots.remove(index);