HTML math tag should have AXRoleDescription as 'math'

According to latest editor's draft http://rawgit.com/w3c/aria/master/html-aam/html-aam.html
HTML math tag should be exposed similar to ARIA role math and hence AXRoleDescription for math tag would be 'math.

This CL depends on https://codereview.chromium.org/722653002/

BUG=429476

Review URL: https://codereview.chromium.org/722663002

Cr-Commit-Position: refs/heads/master@{#304622}

content/app/strings/content_strings.grd

@@ -397,6 +397,9 @@ below:
       <message name="IDS_AX_ROLE_FORM" desc="accessibility role description for form">
         form
       </message>
+      <message name="IDS_AX_ROLE_MATH" desc="accessibility role description for math">
+        math
+      </message>
 
       <message name="IDS_AX_BUTTON_ACTION_VERB" desc="Verb stating the action that will occur when a button is pressed, as used by accessibility.">
         press

content/browser/accessibility/browser_accessibility_cocoa.mm

@@ -614,6 +614,9 @@ - (NSString*)roleDescription {
   case ui::AX_ROLE_MAIN:
     return base::SysUTF16ToNSString(content_client->GetLocalizedString(
         IDS_AX_ROLE_MAIN_CONTENT));
+  case ui::AX_ROLE_MATH:
+    return base::SysUTF16ToNSString(content_client->GetLocalizedString(
+        IDS_AX_ROLE_MATH));
   case ui::AX_ROLE_NAVIGATION:
     return base::SysUTF16ToNSString(content_client->GetLocalizedString(
         IDS_AX_ROLE_NAVIGATIONAL_LINK));

content/test/data/accessibility/math-expected-android.txt

@@ -1 +1,3 @@
-#<skip - blink side patch is required>
+android.webkit.WebView focusable focused scrollable
+    android.view.View
+        android.view.View clickable name='a2+b2'

content/test/data/accessibility/math-expected-mac.txt

@@ -1 +1,8 @@
-#<skip - blink side patch is required>
+AXWebArea AXRoleDescription='HTML content'
+    AXGroup AXRoleDescription='group'
+        AXGroup AXSubrole=AXDocumentMath AXRoleDescription='math'
+            AXStaticText AXRoleDescription='text' AXValue='a'
+            AXStaticText AXRoleDescription='text' AXValue='2'
+            AXStaticText AXRoleDescription='text' AXValue='+'
+            AXStaticText AXRoleDescription='text' AXValue='b'
+            AXStaticText AXRoleDescription='text' AXValue='2'

content/test/data/accessibility/math-expected-win.txt

@@ -1 +1,8 @@
-#<skip - blink side patch is required>
+ROLE_SYSTEM_DOCUMENT READONLY FOCUSABLE
+    IA2_ROLE_SECTION
+        ROLE_SYSTEM_EQUATION
+            ROLE_SYSTEM_STATICTEXT name='a'
+            ROLE_SYSTEM_STATICTEXT name='2'
+            ROLE_SYSTEM_STATICTEXT name='+'
+            ROLE_SYSTEM_STATICTEXT name='b'
+            ROLE_SYSTEM_STATICTEXT name='2'