Convert spellcheck to a buildflag header.

components/spellcheck/spellcheck_build_features.gni

+# Copyright 2016 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Enable the spell checker.
+enable_spellcheck = !is_ios

[groby-ooo-7-16, 2016/11/14 20:05:28]

I'm somewhat unhappy that we control what component a platform includes in
individual build files. Do we have any plans to move to a more central "this
build uses these components" kind of list?

[brettw, 2016/11/14 20:42:53]

No, I'm doing the opposite. Our build files are shared between projects and
things like skia, v8, and webrtc builds (if they haven't forked the build
config) get e.g. ENABLE_SPELLCHECK defines which makes no sense.

The build is a 200K line program, and just as we don't have all C++ globals in
one file, having all build flags is not scalable.

"gn args --list out/Default" will tell you all flags you can override (currently
about 300). These particular flags are not overridable. I almost removed them
and changed the checks to ios conditions, but keeping it seems conceptually
cleaner and we may want to tweak it in the future.

+
+# Use the operating system's spellchecker rather than hunspell.

[groby-ooo-7-16, 2016/11/14 20:05:28]

Note: It might be worth leaving a comment that we *still* use a browser-side
checker if that is turned off, namely the red underline checker. We just don't
invoke the system checker.  Or, possibly, rename this to use_system_checker.

(Yes, I'm not happy about it, either.)

[brettw, 2016/11/14 20:42:53]

Thanks, done.

+use_browser_spellchecker = is_android || is_mac