Fix edge-line artifacts issue on Mali

Fix edge-line artifacts issue on Mali

Some GMs show edge-line artifacts on Mali at the bottom/right of the canvas.

Change the 'rtAdjustment' uniform precision from 'midiump' to 'highp' if the GPU vendor is ARM.

NOTE: Vertex Shader should have default 'highp' precision, but now we explicit declared some uniform/attribute precision as 'midiump'.

This patch should fix these issues:

Issue 3693: Some GMs produce random artifacts on Mali in MSAA4
    https://code.google.com/p/skia/issues/detail?id=3693

Issue 3685: srcmode GM has top/right line artifacts on Mali T604 in MSAA4
    https://code.google.com/p/skia/issues/detail?id=3685

BUG=skia:

Review URL: https://codereview.chromium.org/1154163012