Merge from Widevine repo of http://go/wvgerrit/53625
Passing the third parameter to std::string::insert() as an integer
technically makes for an ambiguous method call, as there are two
overloads that could accept these arguments. While baseline GCC and
Clang discern our intent here correctly, seawardt@ discovered that
XCode is more pedantic and requires that we pass the third parameter
as a char to disambiguate.
Test: WvCdmEnginePreProvTestUat.ProvisioningServiceCertificateInvalidTest
Test: tested as part of http://go/ag/4674759
Change-Id: I65a2506209215cd081c685faac26e08bae486d5e
147f40a5ef