Correct mojo::WrapSharedMemoryHandle usage

Fixes some incorrect uses of mojo::WrapSharedMemoryHandle which
were assuming that the call actually has any control over the memory
protection applied to a handle when mapped.

Where fixing usage is infeasible for this CL, TODOs are added to
annotate follow-up work.

Also updates the API and documentation to (hopefully) improve clarity
and avoid similar mistakes from being made in the future.

BUG=792900

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I0578aaa9ca3bfcb01aaf2451315d1ede95458477
Reviewed-on: https://chromium-review.googlesource.com/818282
Reviewed-by: Wei Li <weili@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Yuzhu Shen <yzshen@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Ken Rockot <rockot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#530268}
void PrintRenderFrameHelper::ComputePageLayoutInPointsForCss(
    blink::WebLocalFrame* frame,
    int page_index,
    const PrintMsg_Print_Params& page_params,
    bool ignore_css_margins,
    double* scale_factor,
    PageSizeMargins* page_layout_in_points) {
  double input_scale_factor = *scale_factor;
  PrintMsg_Print_Params params = CalculatePrintParamsForCss(
      frame, page_index, page_params, ignore_css_margins,
      IsWebPrintScalingOptionFitToPage(page_params), scale_factor);
  CalculatePageLayoutFromPrintParams(params, input_scale_factor,
                                     page_layout_in_points);
}

void PrintRenderFrameHelper::ComputePageLayoutInPointsForCss(
    blink::WebLocalFrame* frame,
    int page_index,
    const PrintMsg_Print_Params& page_params,
    bool ignore_css_margins,
    double* scale_factor,
    PageSizeMargins* page_layout_in_points) {
  double input_scale_factor = *scale_factor;
  PrintMsg_Print_Params params = CalculatePrintParamsForCss(
      frame, page_index, page_params, ignore_css_margins,
      IsWebPrintScalingOptionFitToPage(page_params), scale_factor);
  CalculatePageLayoutFromPrintParams(params, input_scale_factor,
                                     page_layout_in_points);
}
