Merge 281795 "base: Add soft memory limit to DiscardableMemoryMa..."

Merge 281795 "base: Add soft memory limit to DiscardableMemoryMa..."

> base: Add soft memory limit to DiscardableMemoryManager.   
>                                                                                  
> This adds a soft memory limit to the discardable memory manager. Only            
> allocations that haven't been used in specific amount of time are                
> affected by this limit. This allows a client to use a much higher hard           
> limit so temporary spikes in required usage can be handled efficiently           
> without causing an increase in usage while idle.                                 
>                                                                                  
> A "reduce memory usage" function is added to the discardable memory                 
> interface to allow the client to effectively trigger purging of memory           
> that is affected by the soft limit when system reaches what would be             
> considered an idle state.                                                        
>                                                                                  
> BUG=383361, 372158
> TEST=base_unittests --gtest_filter=DiscardableMemoryManagerTest.ReduceMemoryUsage
> 
> Review URL: https://codereview.chromium.org/336273003

TBR=reveman@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=283286

[reveman, 2014-07-16 00:04:49]

Committed patchset #1 manually as r283286.