Add GCAPI to the GN build.

GCAPI is a Windows-specific API for distributors of Chrome.

This adds all chrome/installer targets to "gn check" and fixes many associated issues. The result codes targets are also added to "gn check" to make sure people don't accidentally add stuff to those headers.

Rather than add a disable for the size_t to int warning for this target, I just fixed the warning and removed the disable flag from GYP.

Merged //chrome/common:result_codes into //chrome/common:constants. This is basically a constant and everybody that needed the result codes already depended on the constants target.

Fix google_update build file. The midl template already did everything this file was trying to do. I clarified the template documentation a bit.

BUG=539428

Review URL: https://codereview.chromium.org/1388943004

Cr-Original-Commit-Position: refs/heads/master@{#352869}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f986f9572c792346194055784b9237820ffce52b

toolchain/win/midl.gni

@@ -6,7 +6,8 @@ assert(is_win)
 
 import("//build/config/win/visual_studio_version.gni")
 
-# This template defines a rule to invoke the MS IDL compiler.
+# This template defines a rule to invoke the MS IDL compiler. The generated
+# source code will be compiled and linked into targets that depend on this.
 #
 # Parameters
 #