Fix some properties in autotestPrivate API to camelCase

I just happened to notice that these two properties don't follow
the naming convention.

Right now no one is actually using them (though some Tast tests
are going to use them), so changing their names should be safe
right now.

Bug: 1021353
Test: build passes
Change-Id: I3f578f1a891fdaa68110b4656006dffe1baaed0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894246
Commit-Queue: Jun Mukai <mukai@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712403}

chrome/common/extensions/api/autotest_private.idl

@@ -340,8 +340,8 @@ namespace autotestPrivate {
     DOMString name;
     AppWindowType windowType;
     WindowStateType stateType;
-    Bounds bounds_in_root;
-    Bounds target_bounds;
+    Bounds boundsInRoot;
+    Bounds targetBounds;
     DOMString displayId;
 
     DOMString title;