calls to ClosePlatformFile() ignore the return value, but there are
two calls that do use it:

1. webkit/glue/webfileutilities_impl.cc, closeFile() method: i talked
to jianli who changed that method most recently, and he said he
prefers the handle to be reset to base::kInvalidPlatformFileValue if
ClosePlatformFile() succeeded, so no change is needed to that code.

2. webkit/database/database_tracker.cc, CloseIncognitoFileHandle():
this code needs to be updated.

TEST=WebSQLDBs don't crash in incognito mode.
BUG=56237

Review URL: http://codereview.chromium.org/3387018

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@60685 0039d316-1c4b-4281-b951-d872f2087c98