Fix bug when drawing background tabs in incognito windows with custom themes.

If a theme provided a replacement frame image, but not a replacement incognito
frame or incognito background tab image, the theme service would synthesize one,
but the tab drawing code would not realize this.  As a result it would cache the
appearance of non-hovered background tabs, leading to visual appearance glitches
since not all those tabs should look the same.

BUG=580677
TEST=Install https://chrome.google.com/webstore/detail/tree-branches/bgdeemcfmmabkdhbnhmkhpadancpkgol?hl=en-US , open an incognito window, open several tabs, and ensure the background tab images line up with the frame and the tab appearance doesn't change abruptly when hovering any such tabs.

Review URL: https://codereview.chromium.org/1623643002

Cr-Commit-Position: refs/heads/master@{#371081}

0 files changed, 0 insertions, 0 deletions