Document how mini_installer avoids linking to the CRT

We spent a while figuring out how and why mini_installer.exe avoids
pulling in the C run-time and I wanted to make sure it was working
correctly and was slightly better documented. This just adds some
comments, for future generations.

Bug: 934032
Change-Id: I917a7532d6d8e92a21d58e9cedf0af44fee488ed
Reviewed-on: https://chromium-review.googlesource.com/c/1481956
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634753}

chrome/installer/mini_installer/BUILD.gn

@@ -278,6 +278,9 @@ template("generate_mini_installer") {
     #      dependency on //buildtools/third_party/libc++ in builds that set
     #      use_custom_libcxx=true.
     #
+    # The net result is similar to linking with /NODEFAULTLIB, but more precise
+    # as it just excludes the CRT.
+    #
     # But in asan builds we need to link against the asan runtime library, which
     # in turn depends on the standard library and relies on it to run
     # initializers.

chrome/installer/mini_installer/mini_installer.cc

@@ -14,7 +14,8 @@
 //   BasicRuntimeChecks="0"
 //   BufferSecurityCheck="false" compiler: /GS-
 //   EntryPointSymbol="MainEntryPoint" linker: /ENTRY
-//   IgnoreAllDefaultLibraries="true" linker: /NODEFAULTLIB
+//       /ENTRY also stops the CRT from being pulled in and does this more
+//       precisely than /NODEFAULTLIB
 //   OptimizeForWindows98="1" linker: /OPT:NOWIN98
 //   linker: /SAFESEH:NO
 

chrome/installer/mini_installer/mini_installer_exe_main.cc

@@ -28,8 +28,9 @@ extern "C" int WINAPI wWinMain(HINSTANCE /* instance */,
 }
 #endif
 
-// We don't link with the CRT so we have to implement CRT functions that the
-// compiler generates calls to.
+// We don't link with the CRT (this is enforced through use of the /ENTRY linker
+// flag) so we have to implement CRT functions that the compiler generates calls
+// to.
 
 // VC Express editions don't come with the memset CRT obj file and linking to
 // the obj files between versions becomes a bit problematic. Therefore,