I thought the main problem with porting software to 64-bit was the bad practice of developers assuming the sizes of types. Since this was Google and the project is relatively recent, I'm assuming that is not the reason.
Why couldn't you take the chromium source (and dependencies) and compile them for x64 months ago?
IIRC most issues revolve around plugins and performance (notably V8), possibly not directly because of type sizes but similar low level stuff that hampers JS-to-the-metal optimisations.
Linux, MacOS, and nearly all other Unix-y things use LP64 (int is 32bit, long is 64bit). Windows x86_64 uses LLP64 (int is 32bit, long is 32bit). This can make porting a pain.
Why couldn't you take the chromium source (and dependencies) and compile them for x64 months ago?