Add "depot_tools" to DEPS.

Add "depot_tools" to DEPS, and integrate this into "find_depot_tools".
This formally establishes Chromium's dependency on the "depot_tools"
repository and ensures that Chromium's expectations are versioned
alongside "depot_tools"'s revision.

Add a hook to forcefully disable the DEPS'd "depot_tools"'s self-update
mechanism, which is enabled by default for developers, ensuring that it
will remain at the version pinned in DEPS.

Finally, update "find_depot_tools" logic to point to the new DEPS'd
"depot_tools" directly as a priority preference.

Note that other repositories which have their own versions of
"find_depot_tools" will have to have their scripts similarly updated.

BUG=chromium:727917
TEST=local
  - Ran `gclient sync`, syncs and disables.
  - Manually imported "find_depot_tools", successfully finds DEPS'd
    version.
NOTRY=true

Change-Id: I11355a