Move rtc_task_runner dependency from rtc_base to rtc_base_unittests.

This is step 3 in the task runner migration process started in cl
https://codereview.webrtc.org/2696703009/.

It depends on step 2 being landed in Chrome, cl
https://codereview.chromium.org/2694363005/.

NOTRY=true
BUG=webrtc:6424

Review-Url: https://codereview.webrtc.org/2708843002
Cr-Commit-Position: refs/heads/master@{#16774}

webrtc/base/BUILD.gn

@@ -371,7 +371,6 @@ rtc_static_library("rtc_base") {
   ]
   public_deps = [
     ":rtc_base_approved",
-    ":rtc_task_runner",
   ]
   public_configs = []
 
@@ -893,6 +892,7 @@ if (rtc_include_tests) {
     }
     deps = [
       ":rtc_base_tests_main",
+      ":rtc_task_runner",
     ]
     public_deps = [
       ":rtc_base",