[Pepper] Calculate reserved quota consumption by diff to maximum written offset

Use maximum written offset to calculate the quota consumption.
Calculating it by diff to EstimatedFileSize makes quota calculation very difficult.
This CL turns the strategy more conservative.

BUG=334171

Review URL: https://codereview.chromium.org/143253002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246327 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246327