Fix variable shadowing in Viewer's ImGui code

This worked, but was confusing.

Bug: skia:
Change-Id: I9a387f2fc772277086e0f6ce532c834839b5e4dc
Reviewed-on: https://skia-review.googlesource.com/c/184800
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/tools/viewer/ImGuiLayer.cpp b/tools/viewer/ImGuiLayer.cpp
index 3ed94a0..c6a95b4 100644
--- a/tools/viewer/ImGuiLayer.cpp
+++ b/tools/viewer/ImGuiLayer.cpp
@@ -122,8 +122,8 @@
 
         // De-interleave all vertex data (sigh), convert to Skia types
         pos.rewind(); uv.rewind(); color.rewind();
-        for (int i = 0; i < drawList->VtxBuffer.size(); ++i) {
-            const ImDrawVert& vert = drawList->VtxBuffer[i];
+        for (int j = 0; j < drawList->VtxBuffer.size(); ++j) {
+            const ImDrawVert& vert = drawList->VtxBuffer[j];
             pos.push_back(SkPoint::Make(vert.pos.x, vert.pos.y));
             uv.push_back(SkPoint::Make(vert.uv.x, vert.uv.y));
             color.push_back(vert.col);