Make GN header checker more lenient about toolchains.

The header checker will now automatically accept any headers not known in the
current toolchain. The previous code would do this but required it to not be
present in any toolchain in the entire build.

See the long comment added in header_checker.cc for details.

Adds a dependency fix for the Android build in base.

BUG=

Review URL: https://codereview.chromium.org/1142423004

Cr-Original-Commit-Position: refs/heads/master@{#330839}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a9adafeae2a97bc5a9676b4072ddf8491c212c4d