Alt remapping doesn't work when Shift is pressed on ChromeOS
Alt remapping doesn't work when Shift is pressed on ChromeOS

When the Alt key is mapped to ESC or disabled from the settings, pressing
Shift+Alt will generate a KeySym value of |XKB_KEY_Meta_L/R|. This will
result in the EventRewriter skipping the rewriting of the Alt key completely.
On ChromeOS, the Meta key is not used, and hence we keep it as Alt.

BUG=541468
TEST=tested manually on a link device as well as ChromeOS Linux build.

Review URL: https://codereview.chromium.org/1427743003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#357385}

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