Add missing export to Perfetto glue code

For Windows component builds it is necessary to explicitly share nested
classes if those need to be visible outside of the DLL. This change adds
such an export in order to get Chromium Windows builds with Perfetto
enabled to build.

Change-Id: Ia03383a641425e1eb690bb1f0a539473c83f9a32
Reviewed-on: https://chromium-review.googlesource.com/1063870
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Primiano Tucci <primiano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559501}

services/tracing/public/cpp/perfetto/shared_memory.h

@@ -19,7 +19,8 @@ namespace tracing {
 class COMPONENT_EXPORT(TRACING_CPP) MojoSharedMemory
     : public perfetto::SharedMemory {
  public:
-  class Factory : public perfetto::SharedMemory::Factory {
+  class COMPONENT_EXPORT(TRACING_CPP) Factory
+      : public perfetto::SharedMemory::Factory {
    public:
     std::unique_ptr<perfetto::SharedMemory> CreateSharedMemory(
         size_t size) override;