Revert 197518 "Move download filename determination into a separ..."

It has introduced a number of memory leaks in unit_tests that show up on both
Valgrind and HeapChecker bot and some of them are hard to suppress without
collateral false negatives.

> Move download filename determination into a separate class. 
> 
> * Extracts download filename determination into
>   DownloadTargetDeterminer. The new class maintains state and observes
>   the download. Doing so eliminates the need to pass state around as
>   bound arguments.
> 
> * Guarantees that the completion callback to
>   DownloadManagerDelegate::DetermineDownloadTarget is always invoked,
>   even if the download is interrupted on initialization. This is
>   required for reliably resuming downloads.
> 
> * The DownloadFilePicker always returns the virtual path for downloads
>   to Drive. ChromeDownloadManagerDelegate can use it to keep track of
>   the correct last selected directory for 'Save As' downloads.
> 
> * Since no path subsubstituion is necessary during prompting,
>   DownloadFilePickerChromeOS is no longer necessary.
> 
> * Re-orders the sequence of events so that the user is prompted as early
>   as possible. Expensive history database lookups won't introduce
>   unnecssary jank.
> 
> * History database lookups are only performed if the results of the
>   lookup are necessary.
> 
> * Downloads to Drive don't need the temporary local path to be
>   determined twice when the default downloads directory is Drive.
> 
> BUG=151618
> BUG=104335
> 
> Review URL: https://chromiumcodereview.appspot.com/12850002

TBR=asanka@chromium.org

Review URL: https://codereview.chromium.org/14755003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@197578 0039d316-1c4b-4281-b951-d872f2087c98

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