Convert nonce background calls to async/await (#12853)

The calls to the background in `actions.js` that relate to the custom
nonce feature now use `async/await` and `promisifiedBackground`.

The behaviour should be unchanged except that when setting the nonce
field, the warning is shown in case of error before the loading
indicator drops, which seems like it would be a minor improvement (if
it has any user-facing impact at all).

ui/store/actions.js

@@ -2135,15 +2135,15 @@ export function setUseBlockie(val) {
 }
 
 export function setUseNonceField(val) {
-  return (dispatch) => {
+  return async (dispatch) => {
     dispatch(showLoadingIndication());
     log.debug(`background.setUseNonceField`);
-    background.setUseNonceField(val, (err) => {
-      dispatch(hideLoadingIndication());
-      if (err) {
-        dispatch(displayWarning(err.message));
-      }
-    });
+    try {
+      await background.setUseNonceField(val);
+    } catch (error) {
+      dispatch(displayWarning(error.message));
+    }
+    dispatch(hideLoadingIndication());
     dispatch({
       type: actionConstants.SET_USE_NONCEFIELD,
       value: val,
@@ -2857,19 +2857,17 @@ export function setNextNonce(nextNonce) {
 }
 
 export function getNextNonce() {
-  return (dispatch, getState) => {
+  return async (dispatch, getState) => {
     const address = getState().metamask.selectedAddress;
-    return new Promise((resolve, reject) => {
-      background.getNextNonce(address, (err, nextNonce) => {
-        if (err) {
-          dispatch(displayWarning(err.message));
-          reject(err);
-          return;
-        }
-        dispatch(setNextNonce(nextNonce));
-        resolve(nextNonce);
-      });
-    });
+    let nextNonce;
+    try {
+      nextNonce = await promisifiedBackground.getNextNonce(address);
+    } catch (error) {
+      dispatch(displayWarning(error.message));
+      throw error;
+    }
+    dispatch(setNextNonce(nextNonce));
+    return nextNonce;
   };
 }