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Since the EphemeralTabToolbar's layout has been reused by
PaymentHandlerToolbar. The layout will be more readable if it
uses a more general name.

In this patch, we change the name in the following manners:
* ephemeral_tab_toolbar --> sheet_tab_toolbar
* preview_tab_toolbar_height --> sheet_tab_toolbar_height
* ephemeral_tab_text --> upper_text
* ephemeral_tab_caption --> lower_text

Bug: 999196

Change-Id: Ia993c024f192980d071461f46b73a40106457d50
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1911604
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714704}