Revert of Rename wm_core_export.h to wm_export.h and the wm_core target to just wm...

Revert of Rename wm_core_export.h to wm_export.h and the wm_core target to just wm (https://codereview.chromium.org/237873003/)

Reason for revert:
Seems to have broken compile on the Linux builders:

http://build.chromium.org/p/chromium.linux/builders/Linux%20Builder/builds/29879

ninja -C /b/build/slave/Linux_Builder/build/src/out/Release app_list_unittests aura_unittests base_unittests browser_tests cacheinvalidation_unittests cast_unittests cc_unittests chrome chromedriver_unittests chromium_swarm_tests components_unittests compositor_unittests content_browsertests content_unittests crypto_unittests dbus_unittests device_unittests events_unittests gcm_unit_tests gfx_unittests google_apis_unittests gpu_unittests interactive_ui_tests ipc_tests jingle_unittests media_unittests mojo_apps_js_unittests mojo_common_unittests mojo_js_unittests mojo_public_bindings_unittests mojo_public_environment_unittests mojo_public_system_unittests mojo_public_utility_unittests mojo_service_manager_unittests mojo_system_unittests net_unittests ppapi_unittests printing_unittests remoting_unittests sandbox_linux_unittests sql_unittests sync_integration_tests sync_unit_tests ui_unittests unit_tests url_unittests views_unittests webkit_compositor_bindings_unittests wm_core_unittests -j50
ninja: Entering directory `/b/build/slave/Linux_Builder/build/src/out/Release'
ninja:error: unknown target 'wm_core_unittests'

Original issue's description:
> Rename wm_core_export.h to wm_export.h and the wm_core target to just wm
>
> Extracted from https://codereview.chromium.org/209383003/
>
> http://crbug.com/308710
> TBR=sky@chromium.org
>
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=263812

TBR=sky@chromium.org,ben@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/239023002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@263815 0039d316-1c4b-4281-b951-d872f2087c98