Make it possible for Android GN host builds to use Clang.

Make it possible for Android GN host builds to use Clang.

The problem was that the toolchain definitions had an is_clang block in them, but this is only evaluated in the context of the default toolchain (so false for and Android build). But then when we were re-evaluating the build config for the host compile, it was forcing clang to true which was causing the clang flags to be set.

This changes the way we default the is_clang value. Forcing it to true prevents it from being overridden manually. In some cases, like mac, this is pointless. In other cases like Linux, we may want to turn it on and off. Changing this definition allows us to toggle it on and off for different toolchains as we desire.

Ideally I think the way of defining toolchains would be different to make this a bit more flexible, I'll think about this. But this will work for now.

R=thakis@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=282685

[brettw, 2014-07-10 23:00:11]

To enable for Linux we'd add it to the list of conditions in the is_clang block.
This doesn't work now because the build flags haven't been set up properly. I
was getting clang plugin complaining about virtual destructors in breakpad
files. I assume there's some way that it gets toggled on and off for
chromium-code-vs-not. We'll figure this out in a later step.

[Nico, 2014-07-10 23:06:49]

rslgtm

https://codereview.chromium.org/385823002/diff/20001/build/config/BUILDCONFIG.gn
File build/config/BUILDCONFIG.gn (right):

https://codereview.chromium.org/385823002/diff/20001/build/config/BUILDCONFIG...
build/config/BUILDCONFIG.gn:34: is_clang = (os == "mac" || os == "ios")
The idea here is to add `os == "linux"` here one that compiles?

[Nico, 2014-07-10 23:07:10]

CL description should be "make it _possible_ for …", right?

[brettw, 2014-07-11 20:27:21]

Committed patchset #2 manually as r282685.