[iOS][GN] Port ios_net_unittests to build with gn.

[iOS][GN] Port ios_net_unittests to build with gn.

Add new BUILD.gn file exporting //ios/net and //ios/net:ios_net_unittests
targets (corresponding to ios_net and ios_net_unittests with gyp) and add
dependency on the unit test target in //BUILD.gn.

Add missing dependency on //build/config/mac:compiler when targetting iOS
or mac in //build/config/compiler/BUILD.gn (paralleling the other deps on
//build/config/${system}:compiler).

Change the "-arch" value to target fat architecture "arm64" & "armv7" when
current_cpu is "arm" (better to have fat binaries working on all devices
since it is not possible currently to target "armv7" with gn).

BUG=459705
Committed: https://crrev.com/998e85859261f3a9d1491b6f106b32b5c7dce775
Cr-Commit-Position: refs/heads/master@{#352827}

[sdefresne, 2015-10-06 16:45:18]

sdefresne@chromium.org changed reviewers:
+ dpranke@chromium.org, ellyjones@chromium.org

[sdefresne, 2015-10-06 16:45:19]

dpranke: please take a look at the gn changes
ellyjones: can you review as //ios/net OWNERS

Note that this allow me to build ios_net_unittests using gn instead of gyp (the
target will not run as gn still does not support bundle resources).

$ gn gen --args='target_os="ios" target_cpu="arm"' out/Default-iphonesimulator
Done. Wrote 739 targets from 205 files in 1886ms
$ ninja -C out/Default-iphonesimulator/ ios_net_unittests
ninja: Entering directory `out/Default-iphonesimulator/'
[1895/1895] STAMP obj/ios/net/ios_net_unittests.stamp
$ file out/Default-iphonesimulator/ios_net_unittests.app/ios_net_unittests 
out/Default-iphonesimulator/ios_net_unittests.app/ios_net_unittests: Mach-O
universal binary with 2 architectures
out/Default-iphonesimulator/ios_net_unittests.app/ios_net_unittests (for
architecture armv7):	Mach-O executable arm
out/Default-iphonesimulator/ios_net_unittests.app/ios_net_unittests (for
architecture arm64):	Mach-O 64-bit executable

[Dirk Pranke, 2015-10-06 23:55:45]

lgtm

[Elly Fong-Jones, 2015-10-07 13:22:04]

lgtm

[sdefresne, 2015-10-07 13:22:37]

The CQ bit was checked by sdefresne@chromium.org

[commit-bot: I haz the power, 2015-10-07 13:23:39]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1388983003/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1388983003/1

[commit-bot: I haz the power, 2015-10-07 13:36:56]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-07 13:37:50]

Patchset 1 (id:??) landed as
https://crrev.com/998e85859261f3a9d1491b6f106b32b5c7dce775
Cr-Commit-Position: refs/heads/master@{#352827}