cups_proxy: Add support for the IPP print-color-mode attribute

Some of the printer Tast tests use this attribute.

Bug: none
Change-Id: I07543b408f0282fec6077774bfad91a866b2f4d2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2591914
Auto-Submit: Pranav Batra <batrapranav@chromium.org>
Commit-Queue: Sean Kau <skau@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#837832}

chrome/services/cups_proxy/ipp_attribute_validator.cc

@@ -121,6 +121,9 @@ ValidateAttributeResult ValidateAttribute(ipp_op_t ipp_oper_id,
       {"pages-per-minute", {false, ValueType::INTEGER}},
       {"pages-per-minute-color", {false, ValueType::INTEGER}},
       {"pdl-override-supported", {false, ValueType::STRING}},
+      {"print-color-mode", {false, ValueType::STRING}},
+      {"print-color-mode-default", {false, ValueType::STRING}},
+      {"print-color-mode-supported", {true, ValueType::STRING}},
       {"print-quality", {false, ValueType::INTEGER}},
       {"print-quality-default", {false, ValueType::INTEGER}},
       {"print-quality-supported", {true, ValueType::INTEGER}},