Add EXPORT specifier in /net

To speed up build time on windows component build, I will enable newly added /Zc:dllexportInlines- flag. The flag is similar to -fvisibility-inlines-hidden in gcc/clang.

This flag makes inline member function is not exported if it does not have static local variables. But in few classes of chromium, we need to add additional EXPORT specifier to prevent link failure.

This is a part of effort adding EXPORT speicifer to enable the flag.

Master CL is
https://chromium-review.googlesource.com/c/chromium/src/+/1317069

This CL was uploaded by git cl split.

R=morlovich@chromium.org

Bug: 901709
Change-Id: I1ef0284430ca511ab15dfe53fcf49bfe758a03f7
Reviewed-on: https://chromium-review.googlesource.com/c/1322333Reviewed-by: Maks Orlovich <morlovich@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606488}

net/socket/client_socket_pool_base.h

@@ -427,7 +427,7 @@ class NET_EXPORT_PRIVATE ClientSocketPoolBaseHelper
   // SanityCheck() will always be true, except during the invokation of a
   // method.  So all public methods expect the Group to pass SanityCheck() when
   // invoked.
-  class Group {
+  class NET_EXPORT_PRIVATE Group {
    public:
     using JobList = std::list<std::unique_ptr<ConnectJob>>;