[cleanup] Update enum traits max value for printing ColorModel

Noticed during mojoification preparation for PrintBackend that the max
value for a printing::ColorModel was reflective of an old value that
didn't get automatically adjusted for new additions.

Update to use the value attached to the last color model, to always
be up-to-date.

Bug: 809738
Change-Id: I288ce3a6ab89c3e7ba7eeebb17e78df42f8870dd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2355589Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Alan Screen <awscreen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802306}

chrome/common/chrome_utility_printing_messages.h

@@ -27,7 +27,7 @@ IPC_STRUCT_TRAITS_BEGIN(printing::PrinterCapsAndDefaults)
 IPC_STRUCT_TRAITS_END()
 
 IPC_ENUM_TRAITS_MAX_VALUE(printing::mojom::ColorModel,
-                          printing::mojom::ColorModel::kProcessColorModelRGB)
+                          printing::mojom::ColorModel::kColorModelLast)
 
 IPC_STRUCT_TRAITS_BEGIN(printing::PrinterSemanticCapsAndDefaults::Paper)
   IPC_STRUCT_TRAITS_MEMBER(display_name)