[build] Provide tracing dependency via variable

This avoids introducing an include path outside v8. Instead, the path
to where tracing includes are located is provided via a variable.

Please refer to https://codereview.chromium.org/2068073002.

Once the v8 change makes its way to chromium, we can remove the
TODO in this current change.

BUG=

Review-Url: https://codereview.chromium.org/2061933002
Cr-Commit-Position: refs/heads/master@{#37049}

src/v8.gyp

@@ -387,8 +387,7 @@
       },
       'include_dirs+': [
         '..',
-        # To be able to find base/trace_event/common/trace_event_common.h
-        '../..',
+        '<(DEPTH)',
       ],
       'sources': [  ### gcmole(all) ###
         '../include/v8-debug.h',