Add a DEPS variable that points to src/ for angle

4c984bc7 requires angle to call a script
in build/util/, but this directory is in src/ in chromium, and ./ in
angle.  Similar to how angle_root was defined to point to the right
directory in each project, src_root is added to point to src/ or ./
depending on the project as well.

This is necessary for angle to be able to roll its dependencies forward.

Bug: angleproject:2892
Change-Id: I691d378c76bb238071a59db2298206eba55278bb
Reviewed-on: https://chromium-review.googlesource.com/c/1283830Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600125}

DEPS

@@ -87,8 +87,11 @@ vars = {
   # as an expression.
   'cros_download_vm': '"{cros_board}" == "amd64-generic"',
 
-  # ANGLE's deps are relative to the angle_root variable.
+  # ANGLE's deps are relative to the angle_root variable, except for what's
+  # shared with chromium, such as build/, testing/, etc which are relative to
+  # src_root.
   'angle_root': 'src/third_party/angle',
+  'src_root': 'src',
 
   'android_git': 'https://android.googlesource.com',
   'aomedia_git': 'https://aomedia.googlesource.com',