Building on Windows with gyp fails depending on the result from
sharding the src/v8.gyp:v8_base target. If two source files with the
same name are in the same shard, their output object file path would
conflict with one another. One example of this conflict is v8_base's
runtime/runtime.cc and the V8 inspector's protocol/Runtime.cpp that
is generated at build time, for which the files runtime.obj and
Runtime.obj would be created, but MSVS overwrites one of them with
the other.

Dividing the .obj output path by the original source's extension
prevents this overwrite.

Refs: https://github.com/nodejs/node/pull/13959
Bug: 
Change-Id: I158e6178f2511297899ee50ea159f574916f903f
Reviewed-on: https://chromium-review.googlesource.com/556599Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46354}