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The MacViews browser were missing infobar and background tab assets.
mac_views_browser grit define is added so that we can include infobar assets.
Don't remap theme image ids so that Mac's background tab assets can be
found.
Map IDR_THEME_TAB_BACKGROUND_INCOGNITO to the same asset as
IDR_THEME_TAB_BACKGROUND.

BUG=425229

Review URL: https://codereview.chromium.org/983823004

Cr-Commit-Position: refs/heads/master@{#321602}