observeCpuUsagePromise is used in observeMemoryUsage related methods.
Correct them to observerMemoryUsagePromise.

      --gtest_filter="DiagnosticsApp*"

Bug: 377630459
Test: browser_tests -ozone-platform=headless
Change-Id: Icbc6abfa631dbe17e2c0b5cdafa340715529c26c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5999130


Reviewed-by: Denny Huang <dennyh@google.com>
Commit-Queue: Po-Heng Chen <pohengchen@google.com>
Reviewed-by: Byron Lee <byronlee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1378963}