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The TabContentManager provides APIs to fetch Tab thumbnails from disk,
and we want to reuse these APIs in instant start. Thus, we split the
creation of TabContentManager into two parts:
1. creation: the constructor is called right after the TabModel is
   initialized (pre-native).
2. initWithNative: is called after native is initialized.

Also, add test fetchThumbnailsPreNativeTest() to verify that the
TabContentManager works properly without native initialization.

Bug: 1041865
Change-Id: I22eca912f396acf234118a27348edbd7054cdd29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1992341
Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737114}