Gets gn check to execute cleanly in mojo/common

Gets gn check to execute cleanly in mojo/common

This one is trickier. I'm changing mojo/common:common_base to be a
source_set. Here's why. When I add the necessary deps, in particular
    "//third_party/mojo/src/mojo/public/cpp/bindings:bindings",
then I get link errors for the environment, eg
gen/third_party/mojo/src/mojo/public/interfaces/bindings/interface_control_messages.mojom.cc:719: error: undefined reference to 'mojo::Environment::GetDefaultLogger()'
These symbols are in //mojo/environment. If I add //mojo/environment
to the list of deps for common_base I end up with a cycle as
environment depends upon common_base. By changing to source_set
common_base isn't linked and everyone is happy.

As a result of this I had to consolidate //mojo/environment into a single source set.

BUG=none
TEST=none
R=ben@chromium.org

Committed: https://crrev.com/67b0a7be4ff751b1cf83be5af817a61ff7154aa8
Cr-Commit-Position: refs/heads/master@{#340527}

[Ben Goodger (Google), 2015-07-27 17:58:26]

lgtm

[sky, 2015-07-27 19:12:51]

The CQ bit was checked by sky@chromium.org

[commit-bot: I haz the power, 2015-07-27 19:14:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1256593003/60001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1256593003/60001

[commit-bot: I haz the power, 2015-07-27 19:19:53]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-07-27 19:20:52]

Patchset 4 (id:??) landed as
https://crrev.com/67b0a7be4ff751b1cf83be5af817a61ff7154aa8
Cr-Commit-Position: refs/heads/master@{#340527}

[sky, 2015-07-27 22:20:49]

A revert of this CL (patchset #4 id:60001) has been created in
https://codereview.chromium.org/1262503002/ by sky@chromium.org.

The reason for reverting is: Caused failes on the windows gn bot:
http://build.chromium.org/p/chromium.win/builders/Win8%20GN%20%28dbg%29/build...
:
485/11287] CXX obj/ui/views/views/desktop_screen_win.obj
FAILED: E:/b/depot_tools/python276_bin/python.exe gyp-win-tool link-wrapper
environment.x86 False link.exe /nologo /OUT:ipc_mojo_unittests.exe
/PDB:ipc_mojo_unittests.exe.pdb @ipc_mojo_unittests.exe.rsp
mojo.dll.lib(mojo.dll) : error LNK2005: "public: __thiscall
mojo::common::HandleWatcher::HandleWatcher(void)"
(??0HandleWatcher@common@mojo@@QAE@XZ) already defined in handle_watcher.obj

mojo.dll.lib(mojo.dll) : error LNK2005: "public: __thiscall
mojo::common::HandleWatcher::~HandleWatcher(void)"
(??1HandleWatcher@common@mojo@@QAE@XZ) already defined in handle_watcher.obj

mojo.dll.lib(mojo.dll) : error LNK2005: "public: void __thiscall
mojo::common::HandleWatcher::Start(class mojo::Handle const &,unsigned
int,unsigned __int64,class base::Callback<void __cdecl(unsigned int)> const &)"
(?Start@HandleWatcher@common@mojo@@QAEXABVHandle@3@I_KABV?$Callback@$$A6AXI@Z@base@@@Z)
already defined in handle_watcher.obj

ipc_mojo_unittests.exe : fatal error LNK1169: one or more multiply defined
symbols found.