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

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

Original CL: crrev.com/882673002
Reason for revert: Broke memory.fyi bot (crbug.com/455530)
Reason for reland: The CL was renaming the tracing namespace and missed
                   the rename of the tsan suppression (addressed here)

Original description:

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, 455530
TBR=skyostil@chromium.org,jam@chromium.org,dsinclair@chromium.org,ssid@chromium.org

Committed: https://crrev.com/159604379ff8ef8d3db4577f44d3d6438092ccf0
Cr-Commit-Position: refs/heads/master@{#314806}

[Primiano Tucci (use gerrit), 2015-02-05 12:07:30]

The CQ bit was checked by primiano@chromium.org

[commit-bot: I haz the power, 2015-02-05 12:08:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/869043008/20001

[commit-bot: I haz the power, 2015-02-05 14:20:55]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-02-05 14:21:59]

Patchset 2 (id:??) landed as
https://crrev.com/159604379ff8ef8d3db4577f44d3d6438092ccf0
Cr-Commit-Position: refs/heads/master@{#314806}