Mac: Give app_shim code a nicer home

Mac: Give app_shim code a nicer home

App Shims are small .app bundles for Mac, which run in their own
process to provide packaged apps with an OSX Dock icon, using IPC to
communicate with the browser process.

Most of the app_shim code currently lives in src/apps/app_shim, but app
shims are very much a browser feature, so it doesn't belong there. It's
causing DEPS problems for athena efforts.

This CL moves most of apps/app_shim to chrome/browser/apps/app_shim
(c/b/apps didn't exist until a few months after src/apps/app_shim).

The shim itself (i.e. chrome_main_app_mode_mac.mm) doesn't run in the
browser process - it has its own, shim process. Those parts are moved to
a new folder: src/chrome/app_shim, consolidating files currently spread
across src/apps/app_shim and src/chrome/app.

app_shim_launch.h and app_shim_messages.h are used in both the shim
process and the browser process. They're moved to chrome/common/mac.

Summary of changes:
  {apps => chrome/browser/apps}/app_shim/*
except
  {apps => chrome}/app_shim/chrome_main_app_mode_mac.mm
  {apps/app_shim => chrome/common/mac}/app_shim_launch.h
  {apps/app_shim => chrome/common/mac}/app_shim_messages.h
  app_shim.gypi renamed to browser_app_shim.gypi
  chrome/app_shim/app_shim.gypi added
and
  chrome/{app => app_shim}/app_mode-Info.plist
  chrome/{app => app_shim}/app_mode_loader_mac.mm

- apps/app_shim/DEPS now just generated_resources.h
- 'app_mode_app' target moved from chrome.gyp to app_shim.gypi

BUG=266705

Review URL: https://codereview.chromium.org/585123004

Cr-Commit-Position: refs/heads/master@{#296576}