value_counts not working correctly on (some?) ExtensionArrays
#### Code Sample

```python
pd.Series(list("abcde"), dtype="string").value_counts(normalize=True)
```

```bash
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-40-718821f804b4> in <module>
      1 # lang.value_counts(normalize=True)
----> 2 pd.Series(list("abcde"), dtype="string").value_counts(normalize=True)

~/anaconda/envs/grapy/lib/python3.7/site-packages/pandas/core/base.py in value_counts(self, normalize, sort, ascending, bins, dropna)
   1233             normalize=normalize,
   1234             bins=bins,
-> 1235             dropna=dropna,
   1236         )
   1237         return result

~/anaconda/envs/grapy/lib/python3.7/site-packages/pandas/core/algorithms.py in value_counts(values, sort, ascending, normalize, bins, dropna)
    729 
    730     if normalize:
--> 731         result = result / float(counts.sum())
    732 
    733     return result

AttributeError: 'IntegerArray' object has no attribute 'sum'
```

#### Problem description
The problem seems to be that `value_counts()` on a string extension dtype returns an Int64 dtype, and `sum` is not implemented for IntegerArrays , although it is for Series with ExtensionArrays:

#### Expected Output
```python
vc = pd.Series(list("abcde"), dtype="string").value_counts(normalize=False)
print(vc)
print(vc / vc.sum())
```

```bash
d    1
e    1
b    1
c    1
a    1
dtype: Int64
d    0.2
e    0.2
b    0.2
c    0.2
a    0.2
dtype: float64
```

May be related to #22843?

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 19.2.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8

pandas           : 1.0.3
numpy            : 1.18.1
pytz             : 2019.3
dateutil         : 2.8.0
pip              : 20.0.2
setuptools       : 46.1.1.post20200322
Cython           : None
pytest           : 5.4.1
hypothesis       : None
sphinx           : 2.4.4
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.5.0
html5lib         : None
pymysql          : 0.9.3
psycopg2         : 2.8.4 (dt dec pq3 ext lo64)
jinja2           : 2.11.1
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.5.0
matplotlib       : 3.2.1
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 0.16.0
pytables         : None
pytest           : 5.4.1
pyxlsb           : None
s3fs             : None
scipy            : 1.4.1
sqlalchemy       : 1.3.15
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
numba            : 0.48.0

</details>
