Drop [IsIndex] IDL extended attribute from Speech types

Drop [IsIndex] IDL extended attribute from Speech types as the corresponding
specifications do not mandate that we should throw an IndexsizeError
exception on negative values:
https://dvcs.w3.org/hg/speech-api/raw-file/tip/speechapi.html
http://lists.w3.org/Archives/Public/public-xg-htmlspeech/2011Feb/att-0020/api-draft.html#speech_input_result_list_interface

BUG=237739

Review URL: https://chromiumcodereview.appspot.com/22208002

git-svn-id: svn://svn.chromium.org/blink/trunk@155587 bbb929c8-8fbe-4397-9dbb-9b2b20218538