Add a warning to the top of base/memory/singleton.h

Add a warning to the top of base/memory/singleton.h

BUG=none
TEST=none

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=89870

[darin (slow to review), 2011-06-21 18:47:09]

I like it, thanks!  LGTM

You might also add a warning about keeping singletons trivial or leaf.  It can
become problematic if there are circular dependencies between singletons or if a
singleton does too much work in its destructor.

[scherkus (not reviewing), 2011-06-21 18:54:42]

this time with more warnings!

[commit-bot: I haz the power, 2011-06-21 18:55:51]

Presubmit check for 7217014-4001 failed and returned exit status 1.

Running presubmit commit checks ...

** Presubmit ERRORS **
Found Singleton<T> in the following header files.
Please move them to an appropriate source file so that the template gets
instantiated in a single compilation unit.
  base/memory/singleton.h

[scherkus (not reviewing), 2011-06-21 18:57:47]

On 2011/06/21 18:55:51, I haz the power (commit-bot) wrote:
> Presubmit check for 7217014-4001 failed and returned exit status 1.
> 
> Running presubmit commit checks ...
> 
> ** Presubmit ERRORS **
> Found Singleton<T> in the following header files.
> Please move them to an appropriate source file so that the template gets
> instantiated in a single compilation unit.
>   base/memory/singleton.h

hah! will commit by hand :)