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This CL removes the create flag, and makes the Open operation fail if a file
doesn't exist. This is because a separate event for creating files is coming
soon.

TEST=unit_tests, browser_tests: *FileSystemProvider*Open*, *FileSystemProvider*ReadFile*
BUG=391362

Review URL: https://codereview.chromium.org/379743003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282033 0039d316-1c4b-4281-b951-d872f2087c98