Since there are already x86 emulators in JavaScript, surely Google could implement an ARM emulator running Linux and the Android NDK stack in (P)NaCl? :)
They already have an ARM emulator: The Android emulator. Those who have suffered through it's excruciating performance presumably wouldn't wish that on their worst enemy..
They use qemu in system mode for full-stack emulation, which is, indeed, has performance issues. They may use it in user mode, like shown here: http://tuxthink.blogspot.ru/2012/04/executing-arm-executable... (a first link Google gave me), and implement most of Android APIs in platform-native way not bearing the arm->x86 translation costs. Such approach may provide reasonably better performance.
1. The NDK assumes a Linux environment, which the NaCl sanbox is not.
2. Binaries compiled with the NDK don't meet the special requirements of the NaCl machine code verifier.
3. A lot of NDK-based apps only build the native components for ARM. These wouldn't work on x86 Chromebooks, including the Pixel.