www/chromium: Reduce build log spam

Avoid build log spam by disabling a noisy and purely false-positive warning
in targeted locations. The Clang static analysis of pthread locking
primitives assumes they are not wrapped (as well as relatively simple
locking use). These headers simply wrap the underlying primitives and tickle
hundreds of false positives with deep include stacks that spam build logs.

Approved by: jrm
Differential Revision: https://reviews.freebsd.org/D24000