Make LayerRectList's indexed getter consistent with other indexed getters

Make LayerRectList's indexed getter consistent with other indexed getters:
- The argument is now mandatory
- We no longer throw if the argument is negative (dropped [IsIndex])

This is an internal interface and it has not impact on the existing layout
tests. This is one step towards completely removing [IsIndex] Blink-specific
IDL extended attribute.

R=haraken
BUG=237739

Review URL: https://chromiumcodereview.appspot.com/22184006

git-svn-id: svn://svn.chromium.org/blink/trunk@155536 bbb929c8-8fbe-4397-9dbb-9b2b20218538