drive: Do not rely on argument-dependent lookup when calling MD5DigestToBase16().

drive: Do not rely on argument-dependent lookup when calling MD5DigestToBase16().

Looks like C++ compiler is able to figure out that this function is on
base namespace by deducing this from the dependent type base::MD5Digest.

For more reference on ADL:
http://en.wikipedia.org/wiki/Argument-dependent_name_lookup
http://stackoverflow.com/questions/8111677/what-is-argument-dependent-lookup-aka-adl-or-koenig-lookup

I found this while doing https://codereview.chromium.org/1099453002/.

The error is like the following:

../../chrome/browser/drive/drive_api_util.cc:171:10: error: use of
undeclared identifier 'MD5DigestToBase16'; did you mean
'base::MD5DigestToBase16'?
  return MD5DigestToBase16(digest);
           ^~~~~~~~~~~~~~~~~
           base::MD5DigestToBase16

Log:
http://build.chromium.org/p/tryserver.chromium.linux/builders/linux_chromium_gn_rel/builds/81070/steps/compile/logs/stdio

BUG=None
R=hashimoto@chromium.org

Committed: https://crrev.com/4bbb4bdb858d96d43e32d6d7da2d921f759e2893
Cr-Commit-Position: refs/heads/master@{#326949}

[tfarina, 2015-04-22 20:45:14]

PTAL

[hashimoto, 2015-04-23 04:26:53]

lgtm

[tfarina, 2015-04-24 23:45:56]

The CQ bit was checked by tfarina@chromium.org

[commit-bot: I haz the power, 2015-04-24 23:46:57]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1088593005/1

[commit-bot: I haz the power, 2015-04-25 02:10:24]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-25 02:11:11]

Patchset 1 (id:??) landed as
https://crrev.com/4bbb4bdb858d96d43e32d6d7da2d921f759e2893
Cr-Commit-Position: refs/heads/master@{#326949}