Include <string> and <istream> outside __pnacl__.

Fixes webrtc/ Debug build with libc++ which failed to link std::getline
as this is defined in <istream>. <iostream> causes static initializers
and shouldn't be necessary.

Also adds to README.chromium that the overrides are used too get rid of
static initializers and to build on libc++.

Review URL:

Cr-Original-Commit-Position: refs/heads/master@{#293954}
Cr-Mirrored-Commit: 6455c698e51af65f57a8fe83547296218a5a7251
2 files changed