Revert 246117 "gn: Default is_clang to true on iOS."

Revert 246117 "gn: Default is_clang to true on iOS."

This isn't correct for the xcode iOS builder.

> gn: Default is_clang to true on iOS.
> 
> Mirrors this condition in build/common.gypi:
> 
>       ['OS == "ios"', {
>         # ...
> 
>         # Enable clang and host builds when generating with ninja-ios.
>         'conditions': [
>           ['"<(GENERATOR)"=="ninja"', {
>             'clang%': 1,
>             'host_os%': "mac",
>           }]
>         ],
>       }],
> 
> BUG=336531
> TBR=brettw@chromium.org
> 
> Review URL: https://codereview.chromium.org/143323008

TBR=thakis@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=246144

[Nico, 2014-01-21 22:42:35]

Committed patchset #1 manually as r246144.