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

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@197518 0039d316-1c4b-4281-b951-d872f2087c98

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