content: Fix for GN Mac build.

content: Fix for GN Mac build.

Fix the following error:

ninja: error: '../../content/shell/shell_content_main.cc', needed by 'obj/content/shell/libcontent_shell_framework/shell_content_main.o', missing and no known rule to make it

Tested on Mac with the following command lines:

$ gn gen out-gn/Release --args='is_debug=false is_component_build=false symbol_level=1'
$ ninja -C out-gn/Release

The fix is simple as shell_content_main.cc lives in content/shell/app rather than
content/shell.

BUG=431177
TBR=brettw@chromium.org

Committed: https://crrev.com/c708f3974380273cd9a2c8882715706a0b9e56c1
Cr-Commit-Position: refs/heads/master@{#355993}

[tfarina, 2015-10-25 17:09:31]

TBRing...

[tfarina, 2015-10-25 17:09:37]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-10-25 17:09:49]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1423883002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1423883002/1

[commit-bot: I haz the power, 2015-10-25 17:42:59]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-25 17:43:52]

Patchset 1 (id:??) landed as
https://crrev.com/c708f3974380273cd9a2c8882715706a0b9e56c1
Cr-Commit-Position: refs/heads/master@{#355993}