Fix -Woverloaded-virtual warnings in ICU.

Fix -Woverloaded-virtual warnings in ICU.

1.
In Win32NumberFormat, rename the private format() method to variadicFormat().
It's just an implementation detail of the class. Fixes:
..\..\third_party\icu\source\i18n/winnmfmt.h(146,20) :  error(clang):
        'icu_54::Win32NumberFormat::format' hides overloaded virtual functions
        [-Werror,-Woverloaded-virtual]
    UnicodeString &format(int32_t numDigits, UnicodeString &appendTo, const wchar_t *format, ...) const;
                   ^
..\..\third_party\icu\source\i18n\unicode/numfmt.h(255,28) :  note(clang):
        hidden overloaded virtual function 'icu_54::NumberFormat::format'
        declared here: different number of parameters (4 vs 3)
    virtual UnicodeString& format(const Formattable& obj,
                           ^
...

2.
In Win32DateFormat, remove the two-argument format() function that just
forwards to the base class.  (If this happens to produce compile errors,
the function should be replaced by
`using Win32DateFormat::format(UDate, UnicodeString&) const;`) Fixes:
..\..\third_party\icu\source\i18n/windtfmt.h(55,20) :  error(clang):
        'icu_54::Win32DateFormat::format' hides overloaded virtual functions
        [-Werror,-Woverloaded-virtual]
    UnicodeString& format(UDate date, UnicodeString& appendTo) const;
                   ^
..\..\third_party\icu\source\i18n\unicode/datefmt.h(240,28) :  note(clang):
        hidden overloaded virtual function 'icu_54::DateFormat::format'
        declared here: different number of parameters (4 vs 2)
    virtual UnicodeString& format(const Formattable& obj,
                           ^
...

BUG=505301
R=jshin@chromium.org

Committed: https://chromium.googlesource.com/chromium/deps/icu/+/80ac4c39a5a4fc7fa8ee9dc6b6c057b8b24449e9

[Nico, 2015-08-17 23:54:27]

thakis@chromium.org changed reviewers:
+ jshin@chromium.org

[Nico, 2015-08-17 23:54:27]

Maybe you could upstream this patch?

[jungshik at Google, 2015-08-18 00:01:28]

On 2015/08/17 23:54:27, Nico wrote:
> Maybe you could upstream this patch?

Yeah, I'll if it's still applicable in the ToT upstream. 

LGTM

[Nico, 2015-08-18 00:02:58]

Committed patchset #2 (id:20001) manually as
80ac4c39a5a4fc7fa8ee9dc6b6c057b8b24449e9 (presubmit successful).

[jungshik at Google, 2015-08-25 20:48:24]

On 2015/08/18 00:02:58, Nico wrote:
> Committed patchset #2 (id:20001) manually as
> 80ac4c39a5a4fc7fa8ee9dc6b6c057b8b24449e9 (presubmit successful).

Upstreamed along with the patch : http://bugs.icu-project.org/trac/ticket/11867