Fix a bug that <input type=file multiple> doesn't accept many files....

Fix a bug that <input type=file multiple> doesn't accept many files.

The old code restricted the total characters of selected files to MAX_PATH (256).
It is insufficient for multiple files.  The new code uses UNICODE_STRING_MAX_CHARS
(32767), which is the maximum string length of Win32 API.

Firefox uses 4096.

BUG=44068
TEST=No automated test because it's very hard to test the behavior of
  Windows' file open dialog.
  Manual test: Make two files on a local disk. The length of their names
  should be 200 characters. Visit a page with <input type=file multiple>,
  click the file upload control, and select these two files. The file
  upload control should show "2 files".

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=64037

[Zachary Kuznia, 2010-10-27 07:12:01]

LGTM

[Matt Perry, 2010-10-27 18:19:24]

LGTM.

FYI, TEST= is in fact a description of a manual test, for QA to verify, not to
describe unit tests.