Properly call string fill constructor in win shell integration unittest.
Properly call string fill constructor in win shell integration unittest.

Before it was always creating string of 49 size (value of L'1') filled
with random character based of size of app id string. Found it just
because size after my local changes resulted in inserting forbidden
space ' ' character.

Review URL: https://codereview.chromium.org/1439323003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#359744}

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