Allow GN toolchains to specify runtime deps outputs.

Previously the file considered for the runtime deps computation for linked
targets was just the first output, or, for shared libraries, the
runtime_link_output.

This patch changes that to a list of files for all linker tools. This will
allow us, for example, to be able to automatically bundle symbol files
generated by the build (but not .lib files in the case of shared libraries) to
the swarming bots for testing.

The tool class setters now take objects by copies and move them to avoid extra
copies in many cases. Add a defined_from member to the Tool class.

BUG=631242

Review-Url: https://codereview.chromium.org/2178173002
Cr-Original-Commit-Position: refs/heads/master@{#407852}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 15e0e74f4dbd7616a722702eedd6620b70f50290

toolchain/win/BUILD.gn

@@ -192,6 +192,12 @@ template("msvc_toolchain") {
       ]
       link_output = libname
       depend_output = libname
+
+      # The new version of GN uses a runtime_outputs list rather than a
+      # runtime_link_output string. This needs to be flipped for the next GN
+      # roll.
+      # TODO(brettw) bug 631242: Swap this and remove the old one.
+      #runtime_outputs = [ dllname ]
       runtime_link_output = dllname
 
       # Since the above commands only updates the .lib file when it changes, ask