[fsp] Remove the create flag from the file opening operation.

[fsp] Remove the create flag from the file opening operation.

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
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=282033

[mtomasz, 2014-07-09 07:44:57]

@kalman: PTAL at IDL. (More info for this decision at crbug.com).
@hirono: PTAL at JS and C++.

Thanks.

[hirono, 2014-07-09 09:17:36]

lgtm *.js, *.cc, *.h

[not at google - send to devlin, 2014-07-09 13:31:45]

lgtm

[mtomasz, 2014-07-09 13:52:41]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2014-07-09 13:53:30]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mtomasz@chromium.org/379743003/1

[commit-bot: I haz the power, 2014-07-09 14:35:42]

Change committed as 282033