[Win] Enable control flow guard compiler option (flutter#638)

Enables the /guard:cf flag in the MSVC compile options. This causes the
compiler to analyze control flow for indirect call targets at compile
time, and insert checks to verify the targets at runtime. This flag is a
requirement for Google apps.

Issue: flutter#113119

Author: cbracken

build/config/compiler/BUILD.gn

@@ -76,8 +76,9 @@ config("compiler") {
     # Windows compiler flags setup.
     # -----------------------------
     cflags += [
-      "/GS",  # Enable buffer security checking.
-      "/FS",  # Preserve previous PDB behavior.
+      "/GS",       # Enable buffer security checking.
+      "/FS",       # Preserve previous PDB behavior.
+      "/guard:cf", # Enable control flow guard security checks.
     ]
   } else {
     # Common GCC compiler flags setup.