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Default chunk size is 32KB and it is too small for provided file systems, which
require IPC calls for each chunk. Such small chunks cause the IPC overhead to
slow down the reading process drastically.

Moreover, since there is no streaming support in XMLHttpRequest, returning
data in chunks in providing extensions, requires making separate HTTP requests
with appriopriate Range header. That introduces another overhead.

This patch introduces a wrapper on a FileStreamReader, which converts 32KB
chunk requests to 512KB chunk requests to the underlying file stream  reader.

As a result, the overhead of IPC and HTTP is decreased significantly.

TEST=unit_tests: *FileSystemProvider*BufferingFileStreamReader*
BUG=248427

Review URL: https://codereview.chromium.org/318563002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@282290 0039d316-1c4b-4281-b951-d872f2087c98