[DEPS] Add src-internal revision variable

We plan to move src-internal to a different location (to make it
submodule compatible). Having a variable simplifies the process:
* adding new src-internal entry will be easier - one place to update
  hash
* removing old entry won't produce a conflict should hash be updated

R=estaab@google.com

Bug: 1423117
Change-Id: Ia9176b1f869c7335cd64d9dc63669b4778928425
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4564593


Reviewed-by: Erik Staab <estaab@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Xinan Lin <linxinan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1149406}