gn: bump to latest git HEAD

Support defined(foo["bar"]) syntax.

Since it is already possible to use a string subscript to
get the value of a scope member (i.e. foo["bar"] being the
same as foo.bar, but [] allowing expression evaluation),
this ensures that `defined(foo["bar"])` works as `defined(foo.bar)`
as well.

gn/src/out/last_commit_position.h

@@ -3,7 +3,7 @@
 #ifndef OUT_LAST_COMMIT_POSITION_H_
 #define OUT_LAST_COMMIT_POSITION_H_
 
-#define LAST_COMMIT_POSITION_NUM 2081
-#define LAST_COMMIT_POSITION "2081 (b25a2f8c2d33)"
+#define LAST_COMMIT_POSITION_NUM 2085
+#define LAST_COMMIT_POSITION "2085 (28b7b6c507eb)"
 
 #endif  // OUT_LAST_COMMIT_POSITION_H_