Move tracing namespace from base::debug to base::trace_event.

After having transitioned all the tracing headers from base/debug/ to
base/trace_event, this CL addresses the namespace move.
In principle, this CL should only change the namespace of the
base/trace_event files but the namespace used by the tracing clients.
In order to achieve this, namespace aliases are appended to the
trace_event headers, to make it so that clients can still refer to
base::debug::TraceFoo, with that being aliased to
base::trace_event::TraceFoo.
The upcoming CLs will gradually migrate the clients to use the
base::trace_event namespace and will remove the ns aliases.
Unfortunately, this CL has also to update few tracing clients,
in particular the ones having forward declarations. Forward
declarations, in fact, cannot be aliased as the compiler sees them
before the alias itself.

See crrev.com/837303004 and the related bug for motivations and design doc.

BUG=451032
TBR=skyostil@chromium.org,jam@chromium.org

Review URL: https://codereview.chromium.org/882673002

Cr-Commit-Position: refs/heads/master@{#314657}

1 files changed, 7 insertions, 1 deletions

diff --git a/cc/trees/layer_tree_impl.h b/cc/trees/layer_tree_impl.h
index a58b645..5471565 100644
--- a/cc/trees/layer_tree_impl.h
+++ b/cc/trees/layer_tree_impl.h
@@ -19,10 +19,16 @@
 #include "cc/resources/ui_resource_client.h"
 
 namespace base {
-namespace debug {
+namespace trace_event {
 class TracedValue;
 }
+
+// TODO(ssid): remove these aliases after the tracing clients are moved to the
+// new trace_event namespace. See crbug.com/451032. ETA: March 2015
+namespace debug {
+using ::base::trace_event::TracedValue;
 }
+}  // namespace base
 
 namespace cc {