Move const PortName data out of header file

When non-integral const data is defined in a header file it often ends
up being instantiated in multiple translation units. It also ends up in
the read/write data segment which means it isn't shared between
processes. This change has the following affect on the size of sections
in a 32-bit release Windows build:

chrome.dll
     .text:   144 bytes change
    .rdata:   -32 bytes change
     .data:  -736 bytes change
    .reloc:   140 bytes change
Total change:  -484 bytes

chrome_child.dll
     .text:    32 bytes change
    .rdata:  -112 bytes change
     .data:  -736 bytes change
    .reloc:   -84 bytes change
Total change:  -900 bytes

Note that the sections that increase in size are either shareable
(.text) or discardable (.reloc).

I wish I could use "extern constexpr" to ensure that no constructors are
run, but C++ does not allow that. inline constexpr is not yet supported,
and making the variables a static constexpr member of a class (see
crrev.com/2512753002) seems like overkill and doesn't seem to help.

BUG=630755

Review-Url: https://codereview.chromium.org/2515483002
Cr-Commit-Position: refs/heads/master@{#433727}

mojo/edk/system/ports/name.cc

@@ -8,6 +8,10 @@ namespace mojo {
 namespace edk {
 namespace ports {
 
+extern const PortName kInvalidPortName = {0, 0};
+
+extern const NodeName kInvalidNodeName = {0, 0};
+
 std::ostream& operator<<(std::ostream& stream, const Name& name) {
   std::ios::fmtflags flags(stream.flags());
   stream << std::hex << std::uppercase << name.v1;

mojo/edk/system/ports/name.h

@@ -40,14 +40,14 @@ struct PortName : Name {
   PortName(uint64_t v1, uint64_t v2) : Name(v1, v2) {}
 };
 
-const PortName kInvalidPortName = {0, 0};
+extern const PortName kInvalidPortName;
 
 struct NodeName : Name {
   NodeName() : Name(0, 0) {}
   NodeName(uint64_t v1, uint64_t v2) : Name(v1, v2) {}
 };
 
-const NodeName kInvalidNodeName = {0, 0};
+extern const NodeName kInvalidNodeName;
 
 }  // namespace ports
 }  // namespace edk