There are many ways in Python to make calls to C++ libraries and in C++ you can inline assembly. It's been a while since I used Python, but I remember SWIG being one option: swig.org
SWIG is an interface compiler that connects programs written in C and C++ with scripting languages such as Perl, Python, Ruby, and Tcl. It works by taking the declarations found in C/C++ header files and using them to generate the wrapper code that scripting languages need to access the underlying C/C++ code. In addition, SWIG provides a variety of customization features that let you tailor the wrapping process to suit your application.
C++ I'm even more rusty on, but I do remember writing inline assembler directly in C and C++ files.
In Java you would use JNI to make calls to C++ libraries, typically this is used where a highly optimised C++ library exists which is critical to the performance of the Java application.
How you package it is a different story, a Chrome application typically consists of many files, so you can have compiled or interpreted Python (probably compiled) making calls to compiled C++ libraries that are running separately on their own kernel threads. The Chrome window could be Python while the rendering engine is C++, so each Chrome process you see is potentially one thread running the rendering engine while another thread contains the UI which might be in Python. (This is just speculation from my side and might not be how they actually do it)