Half Life symbol on Portal.exe? [closed]

Portal 1 was originally implemented as a mod for Half Life 2. In fact originally you couldn't buy Portal 1 on its own. It was a free extra on the "Orange Box" - a Half Life 2 compilation which included HL2, the two expansion "Episode 1" and "Episode 2", Team Fortress 2 (also "just" a HL2 extra back then) and of course Portal.

When Valve found that Portal was extremely well-received by critics and gamers alike, they decided to release it as a stand-alone game. They did that by taking the original HL2, removing all the content not needed to run Portal, configure it to start up right into the Portal mod and put it onto Steam. Interestingly, Valve managed to do it in a way that if you install both games through Steam, they will share the files they have in common. When you download Portal without having HL2 installed, it's a 7GB download. When you already have HL2 installed, Steam just needs to download an additional 2.3 GB.

So when you are playing Portal 1, you are technically playing HL2 with custom maps and a custom weapon and bunch of other custom assets. But it all runs in the HL2 executable. That's why the game appears with the Half Life application icon.

Nevertheless, there are a few hints in the games and their promotional material which give the impression that Portal and Half Life do indeed take place in the same fictional universe (or at least that "Black Mesa" and "Apperture Science" exist in both universes). For more information check out the question What are the connections between Portal series plot and Half-Life series plot?