Add comments for struct members that were changed from size_t to uin32_t or uint64_t for IPC safety.

ui/gfx/range/range.h

Index: ui/gfx/range/range.h
diff --git a/ui/gfx/range/range.h b/ui/gfx/range/range.h
index c99ca22041e40eef0398cca027233dea81c557e9..5da71d60c96bb7811a60cb405b015a9fc807fc12 100644
--- a/ui/gfx/range/range.h
+++ b/ui/gfx/range/range.h
@@ -108,6 +108,9 @@ class GFX_EXPORT Range {
   std::string ToString() const;
 
  private:
+  // Note: we use uint32_t instead of size_t because this struct is sent over
+  // IPC which could span 32 & 64 bit processes. This is fine since text spans
+  // shouldn't exceed UINT32_MAX even on 64 bit builds.
   uint32_t start_;
   uint32_t end_;
 };