Revert of Move tracing namespace from base::debug to base::trace_event. (patchset #7 id:120001 of https://codereview.chromium.org/882673002/)

Reason for revert:
broken Memory.FYI TSan bot.
crbug.com/455530

Original issue's description:
> 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
>
> Committed: https://crrev.com/97c5abba36f5ce473cd996fab74fbf5aa9bb5464
> Cr-Commit-Position: refs/heads/master@{#314657}

TBR=dsinclair@chromium.org,picksi@chromium.org,primiano@chromium.org,skyostil@chromium.org,ssid@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=451032

Review URL: https://codereview.chromium.org/904573002

Cr-Commit-Position: refs/heads/master@{#314740}

2 files changed, 2 insertions, 15 deletions

diff --git a/cc/base/math_util.h b/cc/base/math_util.h
index 72f0399..d9cc43b 100644
--- a/cc/base/math_util.h
+++ b/cc/base/math_util.h
@@ -21,17 +21,10 @@
 
 namespace base {
 class Value;
-namespace trace_event {
+namespace debug {
 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 gfx {
 class QuadF;
diff --git a/cc/base/region.h b/cc/base/region.h
index b84dbd5..294ea63 100644
--- a/cc/base/region.h
+++ b/cc/base/region.h
@@ -15,16 +15,10 @@
 
 namespace base {
 class Value;
-namespace trace_event {
+namespace debug {
 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 {
 class SimpleEnclosedRegion;