[jumbo] Replace one of two colliding GetMainFrame with IsMainFrame()

There were two GetMainFrame() functions in chrome/browser which
clashed in jumbo (unity) builds. Since one of them is only
used to check if the current frame is the main frame, simplify
the code a bit.

Change-Id: I2d0b30d4ca2c18f202db709514c55c84a7eb350f
Reviewed-on: https://chromium-review.googlesource.com/c/1378140
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#616720}
1 file changed