The kEnableLowEndDeviceMode flag currently overrides
base::SysInfo::IsLowEndDevice, but does not impact
base::SysInfo::AmountOfPhysicalMemory. Unfortunately, a lot of the code
relies directly on memory queries to turn low-end features, meaning
that we miss out on some low-end-device features.

This change updates AmountOfPhysicalMemory to always return 512MB when
running with --enable-low-end-device.

Additionally, as many low-end device features are enabled in #ifdef
OS_ANDROID blocks, they don't trigger when enabling low-end device mode
on desktop. This change pulls out once such Android-specific tweak,
RGBA_4444 textures, so it can be tested on desktop.

Change-Id: I2865598ba7e1ba800d05aaf62ea7a6720b2ce273
Bug: 743098
Reviewed-on: https://chromium-review.googlesource.com/572121


Reviewed-by: enne <enne@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Eric Karl <ericrk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#489511}