Cossaclabs offers via their Themis framework to run C++ code inside browsers on x86 and now on ARM platforms.
From the article:
Native Client (NaCl) allows browser applications to launch a native low-level code in an isolated environment. Thanks to this, some code, performance code parts can be rewritten in C or C++ easily. Until recent time, NaCl could work on x86-compatible systems only, yet supporting ARM platform becomes very important, because a huge variety of devices (especially the newest Chrome OS laptops), are built on ARM architecture.
All you need to compile the code for ARM is located in the latest Native Client SDK. However, using NaCl forces developers to include support for all used architectures. This is achieved by building NaCl separately for all the architectures supported. Then the browser chooses the correct object to launch, basing on the architecture information.
Despite the fact that ARM architecture support in NaCl is rapidly improving, we should note that while Samsung Chromebooks remain being a primary objective for NaCl developers, it’s too early to talk about adequate ARM architecture support.