fix: use Date as a performance tracker fallback (#7349)

src/tracing/SwapEventTimestampTracker.test.ts

 import { SwapEventTimestampTracker, SwapEventType } from './SwapEventTimestampTracker'
 
 jest.mock('./utils', () => ({
-  calculateElapsedTimeWithPerformanceMark: (mark: string) => {
+  calculateElapsedTimeWithPerformanceMarkMs: (mark: string) => {
     switch (mark) {
       case SwapEventType.FIRST_SWAP_ACTION:
         return 100

src/tracing/SwapEventTimestampTracker.ts

-import { calculateElapsedTimeWithPerformanceMark } from './utils'
+import { calculateElapsedTimeWithPerformanceMarkMs } from './utils'
 
 // These events should happen in this order.
 export enum SwapEventType {
@@ -18,6 +18,7 @@ export enum SwapEventType {
 
 export class SwapEventTimestampTracker {
   private static _instance: SwapEventTimestampTracker
+  private createdAt = Date.now()
   private constructor() {
     // Private constructor to prevent direct construction calls with the `new` operator.
   }
@@ -36,7 +37,7 @@ export class SwapEventTimestampTracker {
 
   public setElapsedTime(eventType: SwapEventType): number | undefined {
     if (this.timestamps.has(eventType)) return undefined
-    const elapsedTime = calculateElapsedTimeWithPerformanceMark(eventType)
+    const elapsedTime = calculateElapsedTimeWithPerformanceMarkMs(eventType, this.createdAt)
     if (elapsedTime) {
       this.timestamps.set(eventType, elapsedTime)
     }

src/tracing/utils.ts

 /**
- * Returns the time elapsed between page load and now.
+ * Returns the time elapsed between page load and now in milliseconds.
  * @param markName the identifier for the performance mark to be created and measured.
  */
-export function calculateElapsedTimeWithPerformanceMark(markName: string): number | undefined {
+export function calculateElapsedTimeWithPerformanceMarkMs(
+  markName: string,
+  fallbackStartTime?: number
+): number | undefined {
   const elapsedTime = performance.mark(markName)
-  if (!elapsedTime) return undefined
-  return elapsedTime.startTime
+  if (elapsedTime) {
+    return elapsedTime.startTime
+  }
+  if (fallbackStartTime) {
+    // On some browsers like iOS WebViews, performance.mark is not supported.
+    return Date.now() - fallbackStartTime
+  }
+  return undefined
 }