Implement use_cfi_cast to optionally enable cast checks.

build/config/sanitizers/sanitizers.gni

Index: build/config/sanitizers/sanitizers.gni
diff --git a/build/config/sanitizers/sanitizers.gni b/build/config/sanitizers/sanitizers.gni
index 32cecffcae1e3ecf9ba178625a873add8a96833a..3cacdf4c0173b08735163f1a2126b206e6710d23 100644
--- a/build/config/sanitizers/sanitizers.gni
+++ b/build/config/sanitizers/sanitizers.gni
@@ -49,6 +49,11 @@ declare_args() {
   # TODO(pcc): Remove this flag if/when CFI is enabled in official builds.
   is_cfi = false
 
+  # Enable checks for bad casts: derived cast and unrelated cast.
+  # TODO(krasin): remove this, when we're ready to add these checks by default.
+  # https://crbug.com/626794
+  use_cfi_cast = false
+
   # By default, Control Flow Integrity will crash the program if it detects a
   # violation. Set this to true to print detailed diagnostics instead.
   use_cfi_diag = false