Revert of Remove tools dir from root webrtc target (patchset #1 id:1 of https://codereview.webrtc.org/2412353004/ )

Reason for revert:
Seems to break our Android Swarming bots. Probably due to https://cs.chromium.org/chromium/src/testing/buildbot/gn_isolate_map.pyl?rcl=0&l=875

Error:
/b/c/b/Android32__M_Nexus5X__dbg_/src/buildtools/linux64/gn gen //out/Debug --check --runtime-deps-list-file=/b/c/b/Android32__M_Nexus5X__dbg_/src/out/Debug/runtime_deps
  -> returned 1
ERROR The label "//webrtc/tools:tools_unittests(//build/toolchain/android:arm)" isn't a target.
When reading the line:
  //webrtc/tools:tools_unittests
from the --runtime-deps-list-file=/b/c/b/Android32__M_Nexus5X__dbg_/src/out/Debug/runtime_deps
GN gen failed: 1

Original issue's description:
> Remove tools dir from root webrtc target
>
> Removing it as we don't need it to build as part of webrtc target.
>
> BUG=webrtc:6412
> NOTRY=True
>
> Committed: https://crrev.com/163b1a2d0a0f8e822d8cd15f6385057bc7988ad1
> Cr-Commit-Position: refs/heads/master@{#14622}

TBR=charujain@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:6412

Review-Url: https://codereview.webrtc.org/2420573002
Cr-Commit-Position: refs/heads/master@{#14624}

webrtc/BUILD.gn

@@ -267,6 +267,7 @@ if (!is_ios || !build_with_chromium) {
       "modules",
       "stats",
       "system_wrappers",
+      "tools",
       "video",
       "voice_engine",
     ]