[fsp] Polish FakeProvidedFileSystem methods.

[fsp] Polish FakeProvidedFileSystem methods.

This patch does two things. (1) Always runs callbacks via a PostTask, (2)
fixes an incorrect expression in EOF detection.

TEST=unit_tests, browser_tests: *FileSystemProvider*
BUG=373165
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=273088

[mtomasz, 2014-05-27 08:07:49]

@kinaba: PTAL at this tiny patch. Thanks.

[kinaba, 2014-05-27 08:48:25]

lgtm

[mtomasz, 2014-05-27 08:49:08]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2014-05-27 08:49:14]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mtomasz@chromium.org/307463002/20001

[commit-bot: I haz the power, 2014-05-27 11:15:04]

FYI, CQ is re-trying this CL (attempt #1).
Please consider checking whether the failures are real,
and report flakes to chrome-troopers@google.com.
The failing builders are:
  android_aosp on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_aosp/builds/7...)

[commit-bot: I haz the power, 2014-05-27 11:19:57]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-27 11:19:58]

Try jobs failed on following builders:
  android_aosp on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_aosp/builds/7...)

[mtomasz, 2014-05-27 23:22:40]

The CQ bit was checked by mtomasz@chromium.org

[commit-bot: I haz the power, 2014-05-27 23:23:31]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/mtomasz@chromium.org/307463002/20001

[commit-bot: I haz the power, 2014-05-28 00:31:37]

Change committed as 273088