To what degree does "Embrace and Extend" apply to what Google is doing?
The code is open; part of the NaCL API is designed around Posix/Linux; the Plugin API is the familiar NPAPI and its extensions.
Which part is proprietary or controlled by Google?
To my mind, some more relevant questions are: Does this tie the web to x86 (and ARM)? Can it be adapted securely to x86-64? Can it be made secure? Can multiple implementations be made secure?
FTA: "a GCC-based compiler for creating x86-32 or x86-64 binaries from C or C++ source code"