gdata: Implement very simple cache eviction logic

The basic idea is to evict files from <gcache>/tmp as needed, but
do not remove recursively as we don't want to touch <gache>/tmp/downloads,
which is used for user initiated downloads like "Save As"

Here's the behaviors added in this patch.

Before downloading a file:
- Check if we have enough space, based on the expected file size.
- If we don't have enough space, remove files in <gcache>/tmp
- If we still don't have enough space, return base::PLATFORM_FILE_ERROR_NO_SPACE
- If we have enough space, start downloading the file

After a file is downloaded:
- At this point, the disk can be full or nearly full for various reasons.
- If we don't have enough buffer space,
  - Remove the downloaded file
  - Remove files in <gcache>/tmp
  - Return base::PLATFORM_FILE_ERROR_NO_SPACE

After a file is unpinned:
- This is a chance to remove files in <gcache>/tmp
- Try to remove files in <gcache>/tmp if we don't have enough space

BUG=chromium-os:28553
TEST=added tests.

Review URL: https://chromiumcodereview.appspot.com/10031001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@131578 0039d316-1c4b-4281-b951-d872f2087c98

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