aria-level is not exposed correctly for html header(h1...h6) tags.
aria-level is not exposed correctly for html header(h1...h6) tags.

This CL will match the implementation on Firefox.

Content:
<h1>Level 1</h1>
<h1 aria-level="2">Level 2</h1>
<h3 aria-level="4">Level 4</h3>
<h4 aria-level="2" role="heading">Level 2</h4>

AX Tree
ROLE_SYSTEM_DOCUMENT READONLY FOCUSABLE
    IA2_ROLE_HEADING name='Level 1' level:1
        ROLE_SYSTEM_STATICTEXT name='Level 1'
    IA2_ROLE_HEADING name='Level 2' level:2
        ROLE_SYSTEM_STATICTEXT name='Level 2'
    IA2_ROLE_HEADING name='Level 4' level:4
        ROLE_SYSTEM_STATICTEXT name='Level 4'
    IA2_ROLE_HEADING name='Level 2' level:2
        ROLE_SYSTEM_STATICTEXT name='Level 2'

Review URL: https://codereview.chromium.org/762103002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@186200 bbb929c8-8fbe-4397-9dbb-9b2b20218538

PeterWangIntel/blink-crosswalk,Pluto-tv/blink-crosswalk,Bysmyyr/blink-crosswalk,Bysmyyr/blink-crosswalk,PeterWangIntel/blink-crosswalk,PeterWangIntel/blink-crosswalk,modulexcite/blink,kurli/blink-crosswalk,nwjs/blink,PeterWangIntel/blink-crosswalk,Pluto-tv/blink-crosswalk,nwjs/blink,PeterWangIntel/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,jtg-gg/blink,modulexcite/blink,XiaosongWei/blink-crosswalk,smishenk/blink-crosswalk,modulexcite/blink,jtg-gg/blink,kurli/blink-crosswalk,modulexcite/blink,kurli/blink-crosswalk,XiaosongWei/blink-crosswalk,PeterWangIntel/blink-crosswalk,smishenk/blink-crosswalk,smishenk/blink-crosswalk,XiaosongWei/blink-crosswalk,XiaosongWei/blink-crosswalk,Bysmyyr/blink-crosswalk,nwjs/blink,smishenk/blink-crosswalk,jtg-gg/blink,Bysmyyr/blink-crosswalk,Bysmyyr/blink-crosswalk,Pluto-tv/blink-crosswalk,smishenk/blink-crosswalk,Bysmyyr/blink-crosswalk,kurli/blink-crosswalk,modulexcite/blink,kurli/blink-crosswalk,Bysmyyr/blink-crosswalk,smishenk/blink-crosswalk,smishenk/blink-crosswalk,Bysmyyr/blink-crosswalk,modulexcite/blink,Pluto-tv/blink-crosswalk,kurli/blink-crosswalk,PeterWangIntel/blink-crosswalk,modulexcite/blink,XiaosongWei/blink-crosswalk,kurli/blink-crosswalk,jtg-gg/blink,modulexcite/blink,jtg-gg/blink,jtg-gg/blink,nwjs/blink,XiaosongWei/blink-crosswalk,Pluto-tv/blink-crosswalk,nwjs/blink,smishenk/blink-crosswalk,Pluto-tv/blink-crosswalk,nwjs/blink,kurli/blink-crosswalk,Pluto-tv/blink-crosswalk,nwjs/blink,modulexcite/blink,kurli/blink-crosswalk,modulexcite/blink,smishenk/blink-crosswalk,Pluto-tv/blink-crosswalk,XiaosongWei/blink-crosswalk,XiaosongWei/blink-crosswalk,Bysmyyr/blink-crosswalk,jtg-gg/blink,smishenk/blink-crosswalk,Pluto-tv/blink-crosswalk,PeterWangIntel/blink-crosswalk,jtg-gg/blink,nwjs/blink,PeterWangIntel/blink-crosswalk,jtg-gg/blink,Pluto-tv/blink-crosswalk,kurli/blink-crosswalk,jtg-gg/blink,nwjs/blink,nwjs/blink,XiaosongWei/blink-crosswalk,Bysmyyr/blink-crosswalk