This is a tracking bug for all Chrome/Android code size improvements brought about by improving the toolchain.

So far:
~1MB+80KB+"about 4%" (likely around 1.8MB) from ARM32 backend improvements (https://bugs.chromium.org/p/chromium/issues/detail?id=481675#c3 and https://reviews.llvm.org/D13650 and http://reviews.llvm.org/D15256)
1.6MB from switch to lld (https://chromeperf.appspot.com/group_report?rev=531064) including 200KB from improved relocation packing (https://bugs.chromium.org/p/chromium/issues/detail?id=760838#c11)
510KB from ARM32 range extension thunk improvements (https://reviews.llvm.org/D44284 and  issue 801601 )
108KB from AArch64 backend improvements (https://reviews.llvm.org/D45199)

Upcoming:
~2MB from enabling ThinLTO (issue 407544)
4.8MB from deduplicating rodata (issue 835622)