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This CL adds support for deleting files and directories, including recursively.
The new event in IDL has [nodoc], since we don't want to show it on
developer.chrome.com before most of the R/W features are done.

TBR=kalman@chromium.org
TEST=unit_tests, browser_tests: *FileSystemProvider*DeleteEntry*
BUG=391362

Review URL: https://codereview.chromium.org/375543002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282605 0039d316-1c4b-4281-b951-d872f2087c98