Add WebUI warmup levels and under each level measure Tab Search startup time

Add an enum WebUIBubbleWarmUpLevel that describes the readiness levels
of the browser prior to showing a new WebUI bubble.

* kNoRenderer:
  No render process is available. No pre-existing process, including
  spare renderers, can be used or reused for this WebUI.
* kSpareRenderer:
  Uses a spare render process for this WebUI.
* kDedicatedRenderer:
  Uses a render process that already hosts other WebUIs prior to this
  WebUI.
* kRedirectedWebContents:
  Uses a WebContents that is initially on a different domain and
  requires redirection to this WebUI.
  This is not currently used anywhere, but worth further investigation.
* kNavigatedWebContents:
  Uses a WebContents that has already navigated to this WebUI.

Add histograms Tabs.TabSearch.TimeToShow.{WarmUpLevel} to record the
startup time of Tab Search under each level. These metrics are similar
to previous metrics but have finer granularity. Specifically,

* The existing WindowTimeToShowCachedWebView2 == kNavigatedWebContents
* The existing WindowTimeToShowUncachedWebView2 is equivalent to an
  aggregation of kNoRenderer, kSpareRenderer and kDedicatedRenderer.

These histogram will help us find the warm-up level that strikes the
right balance between latency and memory overhead in an effort to
improve WebUI startup time.

Bug: 1519190
Change-Id: Ie1889aaf55bd5065d72e50f0524e957e05391022
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5254568
Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@google.com>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1256848}