[iOS] Remove --sysroot from the command-line when targetting iOS.
[iOS] Remove --sysroot from the command-line when targetting iOS.

When targetting iOS, the sysroot is configured using the -isysroot
flag, not the --sysroot, so remove that flags (similar to OS X).

This remove the following from the command-line:
--sysroot=../../../../../../../Applications/Xcode.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator9.2.sdk

BUG=None

Review URL: https://codereview.chromium.org/1851193002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#384807}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk