Fix dropped file handling in WebDragData.

Previously we changed File class constructor so that it specifies
hasBackingFile=false for File backed up by file system URL.

This change broke dropped file handling in WebDragData.

BUG=428623
TEST=WebDragDataTest

Review URL: https://codereview.chromium.org/691993002

git-svn-id: svn://svn.chromium.org/blink/trunk@184700 bbb929c8-8fbe-4397-9dbb-9b2b20218538