Views a11y: Add comment to explain usage of AXNodeData's |state| member.

AXNodeData::state is a bitmask used for setting accessible state flags such as
ui::AX_STATE_DISABLED. It is initialised to 0xFFFFFF, which turns all flags on.
This is done in order to more easily catch bugs where no flags are set at all -
it must be cleared to 0x0 before use. Add a comment explaining this.

BUG=668569

Review-Url: https://codereview.chromium.org/2528103002
Cr-Commit-Position: refs/heads/master@{#435020}

ui/accessibility/ax_node_data.cc

@@ -60,6 +60,8 @@ typename std::vector<std::pair<FirstType, SecondType>>::const_iterator
 AXNodeData::AXNodeData()
     : id(-1),
       role(AX_ROLE_UNKNOWN),
+      // Turn on all flags to more easily catch bugs where no flags are set.
+      // This will be cleared back to a 0-state before use.
       state(0xFFFFFFFF),
       offset_container_id(-1) {
 }