Turn off CFG when not needed

Control Flow Guard is a security feature that can help stop exploits. It
is primarily need on our official builds. On the debug and component
builds where Chromium is primarily developed it just adds a bit of
extra size to the object files.

This was fine until it triggered a spurious link error in component
builds when symbol_level=2 was selected. This wasn't caught by the bots
because most (all?) of them build with symbol_level=1, or non-component.

This turns off CFG for debug and component builds, thus getting those
builds working again for developers.

Bug: 1019970
Change-Id: I7a56af177452e62a5f3ad1daa608970cb9518d4f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1891991Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711083}

build/config/win/BUILD.gn

@@ -89,7 +89,11 @@ config("compiler") {
     # don't emit the CFG checks themselves, but this enables the functions to
     # be called by code that is built with those checks enabled, such as system
     # libraries.
-    cflags += [ "/guard:cf,nochecks" ]
+    if (!is_debug && !is_component_build) {
+      # CFG isn't needed on debug or component builds, adds a bit of bloat to
+      # .obj files, and caused link errors once - https://crbug.com/1019970
+      cflags += [ "/guard:cf,nochecks" ]
+    }
 
     if (is_component_build) {
       cflags += [