Make base::Value::TYPE a scoped enum.

As mentioned in the base::Value refactor proposal, base::Value::Type should be a
C++11 style scoped enumeration. This change addresses this issue by adding the
required keyword, making the necessary substitutions and adding casts where
needed.

TBR=stevenjb@chromium.org
BUG=646113
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel

Review-Url: https://codereview.chromium.org/2539363004
Cr-Original-Commit-Position: refs/heads/master@{#436961}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: dc72ee94d48e9a7db7e681c790fda82ece1bc3e2

tools/gn/command_desc.cc

diff --git a/tools/gn/command_desc.cc b/tools/gn/command_desc.cc
index 9f7f903..1de9fbb 100644
--- a/tools/gn/command_desc.cc
+++ b/tools/gn/command_desc.cc
@@ -56,7 +56,7 @@
       PrintValue(&iter.value(), indentLevel + 1);
       iter.Advance();
     }
-  } else if (value->IsType(base::Value::TYPE_NULL)) {
+  } else if (value->IsType(base::Value::Type::NONE)) {
     OutputString(indent + "<null>\n");
   }
 }