Convert miscellaneous RenderThreadImpl messages to mojom

  ViewMsg_NetworkConnectionChanged
  ViewMsg_UpdateScrollbarTheme
  ViewMsg_SystemColorsChanged
  ViewMsg_SetWebKitSharedTimersSuspended
  ViewMsg_PurgePluginListCache

Some complexity here due to some messages being Mac only and
using Mac-only enum types.

We would still like to avoid any support for preproccessing or
platform-specific declarations in mojom, but this CL does add
support for platform-specific (Mac only atm) typemap
configuration.

BUG=612500

Review-Url: https://codereview.chromium.org/2400313002
Cr-Commit-Position: refs/heads/master@{#426003}
14 files changed