(This is a merge of http://go/wvgerrit/134313.)
This patch fixes code that would trigger -Wshorten-64-to-32 by
implicitly narrowing a variable from 64 to 32 bits. There were two cases
of this in string_conversions.cpp:
* The implicit conversions in DecodeBase64Char() were made explicit,
which required going through both a reinterpret_cast and a static_cast
to express fully.
* a2b_hex() now uses size_t for the length, as it probably always should
have.
Bug: 194971260
Test: x86-64 tests
Change-Id: Ib9715b8adecc104f1a056ab0ac5faa5be74e5e54
921cc2c4a3