clang/win: Fix last plugin warnings in component builds.

Exporting a class on Windows forces instantiation of default constructor,
copy constructor, and sundry similar other functions.  The clang plugin then
complains about these not being out-of-line, even if they're not referred to
from anywhere.  Since having this only on Windows is annoying, we're suppressing
these warnings on exported classes.

In this case however, the inner classes STSState and PKPState aren't exported,
and their copy constructors are only instantiated because their outer class
_is_ exported and has STSState and PKPState members.  This cannot be easily
suppressed, and it's also the only place in the whole codebase where this
happens.

So just export these two structs to make the plugin happy in Windows
component builds.

BUG=467287,483986

Review URL: https://codereview.chromium.org/1124233005

Cr-Commit-Position: refs/heads/master@{#329483}

net/http/transport_security_state.h

@@ -66,7 +66,7 @@ class NET_EXPORT TransportSecurityState
     DomainState();
     ~DomainState();
 
-    struct STSState {
+    struct NET_EXPORT STSState {
       STSState();
       ~STSState();
 
@@ -88,7 +88,7 @@ class NET_EXPORT TransportSecurityState
       std::string domain;
     };
 
-    struct PKPState {
+    struct NET_EXPORT PKPState {
       PKPState();
       ~PKPState();