Documentation: improve FileUtilProxy comment to note about the return value
Documentation: improve FileUtilProxy comment to note about the return value

Most of the methods of FileUtilProxy return false not for operation
failure (since the operation will be performed in async) but if
posting a task to the target task runner has failed.

Added documentation about that in file_util_proxy.h.

BUG=none
TEST=none


Review URL: https://chromiumcodereview.appspot.com/11859025

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@177890 0039d316-1c4b-4281-b951-d872f2087c98

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