ui/base/models: Reverse the DCHECKs of index.
ui/base/models: Reverse the DCHECKs of index.

The way it's written until now is counter-intuitive. This makes it intuitive.

1 - To make sure an int index is in the array range when we are adding we should check for:
index >= 0 && index <= child_count().

So use DCHECK_GE for '>=' and DCHECK_LE for '<=' respectively.

2 - To make sure an int index is in the array range when we are retrieving it from vector we should check for:
index >= 0 && index < child_count().

So use DCHECK_GE for '>=' and DCHECK_LT for '<' respectively.

BUG=None
TEST=app_unittests --gtest_filter=TreeNodeModelTest.*

R=sky@chromium.org

Review URL: http://codereview.chromium.org/7058030

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@87716 0039d316-1c4b-4281-b951-d872f2087c98

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