In building Chromium on ChromiumOS, there are (at least) 3 environments for building that are relevant.
The mechanism for getting API keys into running Chromium on Chromium OS use copy-propagation between the layers.
The enter chroot functionality takes care of propagating the keys. If the file .googleapikeys is not present in the chroot, or is empty, the keys are searched for in your home directory, extracted and written there. It is a line-oriented file that should look like the sample in the chrome page.
To reset these the easiest way to update the chroot is simply:
and then the next time something enters the chroot, your native credentials will be copied in. If the keys are not working, try taking a look at the file to see if the format of your keys outside the chroot confused the propagation.
If you want your chroot to build something non-default or non-shared, you can enter the chroot and change the .googleapikeys directly.
.googleapikeys in the final source build environment. It does this by expanding and converting the file into an include.gypi in the inside environment that does chromium's make.