win: Slightly simplify build flags.

/O2 is the same as /Ox /Gy /GF (and /GF is on by default), so use
/O2 instead of /Ox /Gy.

No behavior change.

Bug: none
Change-Id: Ic77716535d492931e0f5d0422cba36c2ea8e2cdb
Reviewed-on: https://gn-review.googlesource.com/c/3320
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index fde6ced..9f5eaf2 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -334,7 +334,7 @@
 
   elif platform.is_msvc():
     if not options.debug:
-      cflags.extend(['/Ox', '/DNDEBUG', '/GL'])
+      cflags.extend(['/O2', '/DNDEBUG', '/GL'])
       libflags.extend(['/LTCG'])
       ldflags.extend(['/LTCG', '/OPT:REF', '/OPT:ICF'])
 
@@ -348,7 +348,6 @@
         '/D_UNICODE',
         '/D_WIN32_WINNT=0x0A00',
         '/FS',
-        '/Gy',
         '/W4',
         '/WX',
         '/Zi',